Advanced Solar Energy in Space: Part II (Turbines!)

[MatterBeam]

Hi! I've copied over my blog post here, so that you might find it easier to read and comment on.
If you wish to chat with the ToughSF community, I have set up a Discord Channel (Invite link) for you!

Advanced Solar Energy in Space: Part II

 

In this post, we continue looking at high power density options for solar energy.

Brayton cycle

We commonly see the Brayton cycle used to convert heat into work in jet engines and the steam turbines of power plants. There are three main components: a compressor, a heat exchanger and a turbine. Gas is compressed to a high pressure by the compressor, then is heated by the heat exchanger. The turbine expands this gas to convert its energy into the mechanical rotation of the turbine's shaft. The latter is used to compress more gas, with the energy remaining able to be put to work. In a power plant, the remaining energy is used to spin an alternator to generate an electric current. Together, this forms a Brayton-cycle turbo-generator. 

A closed cycle gas turbine adds an additional step: the exhaust gases released by the turbine are recycled back into the compressor. The gas is heated externally and is typically inert, as it is not being burnt up like in an open-cycle gas turbine. This makes it it ideal for space applications. 

Let us have a look at the pressure and temperature conditions at each step of the closed Brayton cycle to understand where the turbine's power is coming from. We assume typical component efficiencies of 80% for the compressor and 90% for the turbine.

We will set Temp1 and Press1 as the temperature and pressure of the gas at the compressor's inlet. Temp 2, Press2 at the compressor's outlet, Temp3, Press3 at the turbine's inlet, Temp4, Press4 at the turbine's outlet. We will use example figures for these to make it easier to understand, in Kelvin and bars respectively. We will assume a monoatomic gas, such as helium, with a constant volume heat capacity of 3.12kJ/kg/K. 

The work cycle of a turbine. The lines are not straight due to inefficiencies.

The gas starts at Temp1: 300K and Press1: 1bar. This is a warm gas at sea level pressure. It contains very little energy: 936kJ/kg.

The compressor raises the pressure to Press2: 10 bars. The gas's temperature increases by a factor determined by the following equation for isentropic expansion:

• Inlet Temperature/Outlet Temperature= Pressure Ratio ^ (1 - 1/y)

The compressor increases the pressure tenfold, to the pressure ratio is 10. 'y' is the adiabatic gas constant. For simple mono-atomic gases, it is equal to 1.6.
We work out that the compressor increases the temperature by a factor 2.37. Temp2 is therefore 711K and the gas contains 2218kJ/kg. Due to the compressor's efficiency of 80%, this step consumes (2218-936)/0.8: 1602kJ/kg.

The gas then arrives at the heat exchanger. It is designed to heat up the gas at constant pressure. The gas exits the heat exchanger at Temp3: 1300K and Press3: 10bar, reaching 4056kJ/kg.The heat exchanger grants an energy increase of (4056-2218): 1838kJ/kg.

The turbine is the most critical component. Here, the gas is expanded back to a pressure of 1bar, so Press4: 1 bar. Isentropic expansion causes a temperature drop. Using the same equation as above, we know that a tenfold pressure decrease will reduce the temperature by a factor 2.37. Therefore, the gas exits the turbine at Temp4: 1300/2.37 = 548K. It falls to 1710kJ/kg. Because the turbine is 90% efficient, it extracts only (4056-1710)*0.9: 2111kJ/kg.

This is exhaust at Temp4 and Press4 must then be cooled down back to initial conditions, of Temp1 and Press1, using a radiator.

The find out the net energy extracted by the turbine, it is easiest to calculate the difference between the energy consumed by the compressor and the energy extracted by the turbine. This amounts to 2111-1602: 509kJ/kg.

Compare this to the energy granted by the heat exchanger to find the overall efficiency.

509/1838: 0.2777 or 27.8%

Technically, gas pressure also contains energy, but in a closed cycle, what is spent in the compressor to increase the pressure is regained in the turbine when it expands the gas.

 

To extract more power and increase overall efficiency, we can immediately understand that we need a higher temperature gradient. As we have seen in Part 1 of Advanced Solar Energy in Space, it is possible to design a heat exchanger that reaches thousands of Kelvin under concentrated sunlight. We will use this as the heat source for our space-based turbogenerator, as has been considered before by NASA.

However, we must make sure that the turbine blades are able to survive the Temp3 conditions. While there are many materials that do not melt even at very high temperature, there are few that remain strong enough to rotate at thousands of RPM without deforming under those same conditions. 

Strength/Temperature curve for a Nickel superalloy.

Shown above is the yield strength of a Ni-Cr-W-Al-Ti-Fe-Si-C-B (Nickel, Chromium, Tungsten, Aluminium, Titanium, Iron, Silicon, Carbon, Boron) superalloy. It is designed to survive for 100000 hours under conditions of high stress (1000 bars) and high temperature (1023K). Looking at the graph, we notice that it retains most of its strength up to about 750^oC, making it ideal for the conditions it was designed for. However, if we used it in the turbine we used as an example above, operating at 1300K (1027^oC), it would have a strength reduced by over 80% compared to what it was designed for!

Blades must survive both centrifugal forces and thermal stresses.

The choice of materials is therefore critical in designing a turbogenerator. 

Silicon Carbide is a good material choice, used in jet turbines at up to 1700K temperatures. Nickel-based superalloys are another option, retaining their strength at up to 70% of their melting temperature, but they are much denser.

Tungsten-based alloys would obtain the best strength at 2273K+ temperatures. It is very dense though, which would increase turbine mass.

If we want even higher temperatures, we need to use active cooling of the turbine blades and ceramic materials (such as hafnium carbide). This allows us to reach 2500K or better operating temperatures. 

There are also other considerations. 

Only the first turbine stage really handles high temperatures.

We aim for a high specific power. This means reducing the mass of the equipment required to handle a certain level of heat input while maximizing efficiency by using a high temperature gradient. High pressure ratios are therefore also necessary, as they allow a large pressure (and temperature) drop in the turbine. 

We also want a higher rather than lower turbine exhaust temperature. This is because all waste heat must be radiated using radiators, and their performance rises by the power ^4 with increasing operating temperature. For example, if the turbine releases gases at 600K instead of 400K, it allows for radiators that are over five times smaller and lighter.  Modern Brayton cycle example

We will be using existing technology for this example.

Solar receiver.

As in previous 'modern' examples, we will be using a concentrated solar power set-up, where large parabolic surfaces of a thin, reflective material, such as Mylar, is used to focus sunlight onto a solid heat exchanger.

A 10000x solar concentration means that 10000m^2 of reflectors will focus sunlight onto 1m^2 of heat exchanger. 

Possible configuration of solar concentrators.

Mylar is 98% reflective. If the reflectors have a mass of 7 grams per square meter, as solar sails have demonstrated, then this means that 70kg of reflectors will deliver 1367*0.98*10000: 13.4MW of solar power at an average power density of 191.4kW/kg. 

The heat exchanger we will be using is made of tungsten, and we will heat it to a temperature of 2500K. It has a very high heat tolerance and high emissivity. It also has the strength to survive a high pressure flow using thin channel walls, which reduces the overall mass of the heat exchanger. 90% efficiency is expected, with 12MW of heat absorbed and the remainder re-radiated.

We will be considering a square grid of microchannels. The grid walls are thin tungsten maintained at 2500K. To maintain a constant pressure while heating the gases, the grid must be contained inside nozzles inspired by turbojet burners. 

1mm thick heat exchanger fins spaced by 1mm allows for a very large effective surface area in a small, lightweight volume. The fins will mass 19.3kg/m^2. The average distance between the gas and the fins is 0.5mm.

The gas that will flow through the turbine will be a 50/50 mixture of Xenon and Helium. The Xenon makes the gas denser, which reduces the turbine rotation velocity, so the strength requirements of the turbine blades is lower and therefore makes for a lighter turbine. Helium has a high thermal conductivity, which allows for smaller heat exchanger. Both are inert, so there is no fear of oxidation of the turbine materials. It has a molar mass of 67g/mol, a heat capacity of 2596J/kg/K and a thermal conductivity of 0.29W/mK (Xenon does not contribute much), based on figures from here.

Using the this distance, the temperature gradient and the thermal conductivity of the gas mix, we can calculate the heat transfer rate.

• Heat transfer rate: Thermal conductivity * Temp. Gradient/Fluid thickness

Heat transfer rate is in W/m^2. Thermal conductivity is in W/mK, temperature gradient in Kelvin and fluid thickness in meters. To calculate this figure, we first need to find the initial and final temperature of the gas mix as it travels through the heat exchanger.

The initial temperature here will be the temperature of the gas after leaving the compressor. The final temperature will be the maximum temperature the turbine materials can handle. 

As will be calculated below, the initial temperature will be 484K and the final temperature will be 1600K. This means that the heat transfer rate is an average of 1170kW/m^2 (484K) and 522kW/m^2 (1600K), or 846kW/m^2.

Using the heat capacity of the helium-xenon mix, we can determine that 5.18kg/s mass flow rate is required to absorb the 12MW of heat. The surface area to do is 12000/846: 14.18m^2, which will mass 271kg.

The heat exchanger's power density is therefore 12000/271: 44kW/kg. 

Let us now design the compressor.

We will be using an axial multi-stage transonic compressor. They are suitable for our purposes as they are very efficient and will operate in a single, carefully controlled environment.

Based on this paper.

Each stage of the compressor increases the pressure of the gas mix by a certain ratio. For efficient subsonic designs, this can be at most 2.1. The effect is compounded by the number of stages. 

Based on the equation for isentropic expansion, we can assert that a higher pressure ratio allows for higher efficiency, as the temperature gradient in the turbine will be greater. We therefore aim for a pressure ratio of 30.

To achieve this pressure ratio, the compressor must contain (30^(1/2.1)): 5 stages. If the initial pressure is 1 bar, the final pressure is 30 bars. Normally, if the initial temperature is 300K, the gases exiting the compressor would be heated by a factor 30^(1-1/1.66): 3.866 to 1160K.

There is little margin between 1160K and the maximum operating temperature of modern turbine materials. The gas would not absorb much energy, so a large mass flow rate is required, which would lead to a larger compressor that consumes even more energy.

The solution is one employed by actual high pressure ratio engine: to split the compressor into low pressure and high pressure sections, and to cool the gases in between.

We will therefore split the 5 compressor into two parts, with an intercooler. The first part is three low pressure stages (1 bar to 9 bar) that raise the temperature from 300K to 718K. It is followed by an intercooler that reduces the gas temperature from 718K back down to 300K. The second part is two high pressure stages (9 bar to 30 bar) that raise the temperature from 300K to 484K. 

There is a significant difference between 484K and 1160K!

Using the energy contained in the gases, we can calculate the work done by the compressor. The low pressure compressor raises the gas temperature from 300K (779kJ/kg) to 718K (1864kJ/kg). Since the mass flow rate is 2.89kg/s, this translates to a power consumption of 3.14MW. The intercooler then has to get rid of 3.14MW of heat. The high pressure compressor raises the gas temperature from 300K to 484K (1256kJ/kg), which requires an input of 0.48MW. 

We can expect an 80% efficiency from the compressors, so the total power consumption of the compressors is 4.53MW.

Next is the turbine stage.

Based on Silicon Carbide composites developed by NASA's Glenn Research Center, we can expect a turbine to operate at 1600K without requiring any active cooling. 

Turbines expand the gases they receive in multiple stages. Pressure compounding turbines lower the gas pressure without changing the gas velocity much. 

Pressure compounding impulse turbine.

We will use a turbine pressure ratio of 5.5. Only two stages are required to expand the gas from 30 bars back down to 1 bar. 

The temperature drop that accompanies that pressure change is by a factor 30^(1-1/1.66): 3.866, from 1600K to 413K.

The turbine reduces the thermal energy in the gas from 4153kJ/kg (1600K) to 1085kJ/kg (418K). It therefore should extract 8.87MW from the 2.89kg/s gas flow. However, we should only expect about 90% efficiency, for an actual figure of 7.98MW. 

The exhaust gases must then be cooled down from 418K to 300K, at a rate of 0.88MW. 

If we add up the power generated and consumed, we obtain a net figure of 3.45MW.

To move onto calculating an estimate of the turbine's mass, we must first estimate its dimensions. The helium-xenon gas mixture will exit the heat exchanger at Mach 1 (choked flow). The speed of sound in this mixture is 578m/s. The mass flow rate translates into a volume flow rate of 0.194m^3/s. An inlet area of 0.000336m^2 is needed to allow this flow to pass, which is a disk 0.021m wide.

However, looking at actual turbine designs, the exposed blades on the first turbine stage only represent a fraction of the total radius. The blade-to-hub ratio can be as low as 10%. This means that the first stage of the turbine is 10% blade (where gas flow) and 90% hub, so the total width is increased ten-fold and the total area a hundred-fold. The smallest turbine stage is therefore 0.21m wide for 0.336m^2. 

The final stage expands by a ratio equal to the pressure ratio, like any nozzle. Therefore, the second stage must have a surface area 5.5 times larger, or 1.85m^2. This requires a width of 1.54m. 

The average width of our turbine is 0.87m.

The LHTEC CTS800 is representative of modern high-performance turboshaft engines, using composite materials and being optimized for light weight. It has a similar number of stages as our design, and is 1.5 times as long as it is wide. Its density is roughly 200kg/m^3. Using these numbers, we can expect our turbine to be 159kg. The 'modern' turbine alone has a power density of 21.7kW/kg.

The turbine's shaft is connected to an electrical generator that converts mechanical energy into electricity. 7kW/kg has already been tested using current technology, at an efficiency of over 95%. The operating temperature is as high as 370K.

308mmx150mm, 24.4kg, handles 170kW.

Using that generator to handle 3.45MW of mechanical power would require a mass of 493kg. It would produce 3.28MW of electricity and 173kW of waste heat.

After the turbine and the generator comes the waste heat management systems. There are three sources of waste heat: the compressor intercooler (3.14MW@718K), the turbine exhaust (880kW@418K) and the generator (173kW@370K).

A design with multiple radiators at different temperatures.

We will use three sets of radiators:
-a graphite fin radiator for the intercooler, at 718K.
-a magnesium fin radiator painted black for the turbine, at 418K.
-a magnesium fin radiator painted black for the generator, at 370K.

The high thermal conductivity of graphite and magnesium allow us to do away with any coolant loops within the radiator, which makes then very thin and with a low mass per area. Instead, coolant loops exchange heat with the fins at the attachment point at the base of the fins.

Black paint, such as a micrometer thick vapor-deposited layer of graphite flakes, is necessary to increase the emissivity of magnesium closer to 0.98.   

1mm thick graphite fins work out to 2.3kg/m^2. At 718K, the radiators remove 29535W per square meter (double-sided). Handling 3.14MW of waste heat requires 106m^2 of radiators with a mass of 245kg.

1mm thick magnesiums fins have 1.7kg/m^2. At 418K, the radiators remove 3392W per square meter. 880kW of waste heat requires 259m^2 of radiators with a mass of 441kg.

The same magnesium fins at 370K remove 2083W per square meter. 173kW of waste heat requires 83m^2 of radiators with a mass of 141kg.

If we put together all these components, we obtain an output of 3.28MW of electrical power produced by 70kg of solar collectors, 271kg of heat exchanger, 159kg of turbines, 493kg of generator and 827kg of radiators for a total of  1820kg. The system power density ends up as 1.8kW/kg, although realistically it will be lower.

Advanced Brayton cycle example

We will now consider a turbogenerator made using more advanced materials and techniques. 
 Let's start with the same solar collector surface area of 10000m^2. However, the reflective surfaces will mass only 1 gram per square meter, for a mass of 10kg.

The critical difference between the 'advanced' and 'modern' designs is the use of carbon materials currently only being tested in laboratory environments. One such material is graphene. It has an exceptional thermal conductivity. By using a thin layer of graphene on vitreous carbon, it can also gain the strength of diamond while surviving very high temperatures.

Compressed vitreous carbon weave.

This allows us to design a heat exchanger that supports higher temperatures and higher pressures with even smaller walls. We will operate the heat exchanger at 3500K. Efficiency will depend on the ratio between sunlight intensity absorbed and heat re-radiated, which will set for 95% efficiency. As before 12MW of heat is absorbed.

The plate fins will be as thin as 10 micrometers. This reduces the mass per area to 15 grams per m^2.

We will aim for a gas to reach a temperature of 3000K while leaving the heat exchanger. At such high temperatures, typical helium mixes reach extreme velocities, which would force the turbine to spin at unsustainable rates. One method of reducing the gas velocity is to increase its molar mass. 

Therefore, Mercury is ideal. It has a very high molar mass of 200g/mol, so it will be 200/67: 3 times slower at the same temperature as the 67g/mol Helium-Xenon mix from the previous design. Mercury boils at just 357K, so it is unlikely that it will condense at the turbine exit. 

However, the metal has a relatively poor thermal conductivity in the gaseous state, at roughly 0.02W/mK, based on this table. To compensate for this, we will use a very narrow spacing between the heat exchanger's channel walls, at 10 micrometers. The average distance between the gas and the walls falls to 5 micrometers.

The heat transfer rate in this micrometer-scale heat exchanger ranges from 2MW/m^2 at 3000K to 8.37MW/m^2 at 1208K. On average, it equals 5.18MW/m^2. The heat exchanger for 12MW of power will mass 0.034kg. 

We will now work on the compressor.

Carbon fibre is ideal for compressors. It is very strong yet lightweight, meaning that fan blades and disks can reach very high RPMs. Centrifugal compressors become competitive even in large diameters. Their downside is that they become rather inefficient when used in multiple stages. The trick is to reach a very high pressure ratio within a single stage: this requires the use of supersonic centrifugal compressors.

Gas flow simulation in a centrifugal compressor.

Current test bed compressors achieve a pressure ratio of 12 with an efficiency of 90%. Improvements are likely, but we will use these numbers.

The Mercury is kept at 450K to prevent condensation. It enters the compressor at a pressure of 1 bar. At the exit, it reaches a pressure of 12 bar. As Mercury is monoatomic, the temperature increases by a factor 12^(1-1/1.66): 2.68, up to 1208K.

Supersonic impeller.

Mercury vapour has a heat capacity of 872J/kg/K. It enters containing 261.7kJ/kg (450K), and exits with 1053.7kJ/kg (1119K). The work done is 792kJ/kg.

To transport 12MW of heat, a mass flow rate of 12000000/((3000-1208)*872): 7.67kg/s. The compressor therefore inputs 6.08MW of work. At 95% efficiency, it consumes 6.4MW.

The turbine must expand this flow.
A radial turbine of equal pressure ratio is ideal, although thanks to high molecular weight gases, much higher is achievable. 

Single stage centrifugal compressor with radial turbine.

A pressure ratio of 12 allows for a temperature drop by a factor 2.68. The mercury gas is expanded from a temperature of 3000K to 1119K. It initially has 2616kJ/kg (3000K), and exits with 975kJ/kg (1208K), so the turbine extracts 1640kJ/kg.

At a mass flow rate of 7.67kg/s and 95% efficiency, this works out as a turbine output of 11.95MW.

The net power generated by the turbine is 11.95-6.4: 5.55MW

The combination of centrifugal compressor and radial turbine is commonly found in automobile turbochargers. 

At the inlet, this turbine accepts 7.67kg/s of mercury at 450K. This corresponds to a volume flow rate of 1.435m^3/s. We expect this design to mass roughly 2kg, based on approximations relative to turbochargers that accept similar volume flow rates and factoring in the specific strength of the carbon materials compared to steel and nickel alloys more commonly used.

As the turbine shaft spins, it drives an electric generator. Superconducting magnets offer very high magnetic field strengths and zero-resistance current flows.
While achieving superconductivity is difficult with current magnets due to the extremely low temperatures that must be achieved (2-4K), there is research currently being done on high temperature superconductors based on copper oxide ceramics (BSCCO and YBCO). Mercury-Barium-Calcium cuprate HBSCCO can operate at temperatures as high as 133K. 

Many high temperature superconducting (HTS) electric generators are currently under development.

American Superconductor Corporation and Northrop Grumman 36.5MW HTS motor.

NASA research into electric aircraft that can compete in terms of output and specific power with conventional turbines provides estimates for the power densities achievable with superconducting generators: 80kW/kg or more, at 99.98% efficiency, for use on the N3-X hybrid-wing airliner.
 

Using carbon materials for the rotors, and statene for wiring, could easily increase this power density to 100kW/kg. With these materials, handling 5.55MW of power requires a generator mass of 55.5kg and would produce 1.11kW of waste heat at 100K. 

The total amount of waste heat to be radiated out of the system is 5.47MW from the exhaust gases at 1208K, and 1.11kW from the generator at 100K.

The exhaust gas heat can be handled by a wire radiator. 

Carbon fibres.

Thanks to carbon materials such as carbon fibre, we can design the radiating wires to have a high thermal conductivity, high emissivity and high strength despite being very thin.

1 micrometer wires will be used with 1750kg/m^3 density and 0.98 emissivity. They are exposed to space at 1208K. Each meter length of carbon wire has a mass of 1.375 nanograms and a surface area of 3.14*10^-6 m^2. 

One million such wires can be aligned in parallel in a 1m^2 space. Interreflection reduces radiative efficiency to 70.4%. This means each square meter of microwire radiator has a mass of 1.375 grams and an effective area of 2.21m^2. At 1208K, it radiates 118.33kW/m^2. So, the power density of the microwire radiator is 190.2MW/kg.

The radiators for the turbine exhaust will mass 0.0292kg.

If those same wires were used to cool the cryogenic generators at 100K (5.6W/m^2), we would need a mass of 0.124kg.

We can now add up the mass of all these components to work out an estimate of the system power density. The advanced turbine design produces 5.55MW of power for a mass of 57.69kg, thereby achieving a power density of 96kW/kg.

Next, we will look at the Rankine cycle, as well as some rather exotic power generating schemes.

[PB666]

This blog needs a major rewrite.

[MatterBeam]

19 minutes ago, PB666 said:

This blog needs a major rewrite.

The entire blog? What would you suggest?

[Starman4308]

6 hours ago, PB666 said:

This blog needs a major rewrite.

Not even remotely helpful critique. There is precisely no indication of what he might improve on, what he's done well, etc.

6 hours ago, MatterBeam said:

We can now add up the mass of all these components to work out an estimate of the system power density. The advanced turbine design produces 5.55MW of power for a mass of 57.69kg, thereby achieving a power density of 96kW/kg.

Impressive. I suspect it's an optimistic estimate, but it's still a quite significant improvement over PV cells.

A few figures for PV panels that I've found:

Juno solar panels: About 0.04 kW/kg

Wikipedia's estimate for current state-of-the-art: About 0.077 kW/kg

SLASR (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.2067&rep=rep1&type=pdf) : ~ 0.36 kW/kg

Ultrathin perovskite (without ancillary equipment such as the support structure: https://www.nature.com/articles/nmat4388): 23 kW/kg (PV cells only)

It may also be possible to extract additional energy via thermocouples attached to the back of the panel, extracting electricity from the heat rejection system.

 

Overall, based on the SLASR results, I think it would be realistic to hit 0.5 kW/kg for PV cells, possibly in a hybrid PV/TEG arrangement. The relatively conservative 1.8 kW/kg figure is still significantly higher than this, and I'm given to understand the final 96 kW/kg figure is based on a fair number of technologies that are nowhere near production ready... and while I could be mistaken, there's a couple things I'm not sure of for that*.

*First, 10 micrometer channels for gas could produce significant flow issues, and might not work as expected. Second, I'm not sure you actually accounted for the mass of the mercury vapor used in the system.

What the analysis suggests to me is that a solar-powered turbine could produce more power per kilogram than PV cells, at the cost of far more moving parts, and possibly being more sensitive to reduced solar insolation. There's a few scenarios where this could be useful. First, if the solar turbine is less sensitive to the van Allen belts than PV cells, one could imagine using those instead of PV cells to power ion engines out of LEO. Second, it could be used for power-intensive applications such as driving ion engines, with PV cells used during coast phases to reduce wear on the moving parts of a turbine.

[YNM]

Do you realize that heating things up is the least efficient of methods ? Also, how will the "gas" (working fluid) cool back down ? I don't think wire radiators.

[MatterBeam]

19 hours ago, Starman4308 said:

Not even remotely helpful critique. There is precisely no indication of what he might improve on, what he's done well, etc.

Impressive. I suspect it's an optimistic estimate, but it's still a quite significant improvement over PV cells.

A few figures for PV panels that I've found:

Juno solar panels: About 0.04 kW/kg

Wikipedia's estimate for current state-of-the-art: About 0.077 kW/kg

SLASR (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.2067&rep=rep1&type=pdf) : ~ 0.36 kW/kg

Ultrathin perovskite (without ancillary equipment such as the support structure: https://www.nature.com/articles/nmat4388): 23 kW/kg (PV cells only)

It may also be possible to extract additional energy via thermocouples attached to the back of the panel, extracting electricity from the heat rejection system.

 

Overall, based on the SLASR results, I think it would be realistic to hit 0.5 kW/kg for PV cells, possibly in a hybrid PV/TEG arrangement. The relatively conservative 1.8 kW/kg figure is still significantly higher than this, and I'm given to understand the final 96 kW/kg figure is based on a fair number of technologies that are nowhere near production ready... and while I could be mistaken, there's a couple things I'm not sure of for that*.

*First, 10 micrometer channels for gas could produce significant flow issues, and might not work as expected. Second, I'm not sure you actually accounted for the mass of the mercury vapor used in the system.

What the analysis suggests to me is that a solar-powered turbine could produce more power per kilogram than PV cells, at the cost of far more moving parts, and possibly being more sensitive to reduced solar insolation. There's a few scenarios where this could be useful. First, if the solar turbine is less sensitive to the van Allen belts than PV cells, one could imagine using those instead of PV cells to power ion engines out of LEO. Second, it could be used for power-intensive applications such as driving ion engines, with PV cells used during coast phases to reduce wear on the moving parts of a turbine.

@Starman4308 Thank you for taking the time to comment.

Advanced Solar Energy in Space: Part I focuses on photovoltaics, which you might find it interesting.
Thermocouples allow you to recover some of the energy that would otherwise be lost as waste heat... but they also produce waste heat at a lower temperature. Lower temperature heat is harder to get rid of using radiators, so you would need much larger radiating surfaces, which are consequently heavier. The mass gain from dealing with lower temperature waste heat usually offsets the extra output from using thermocouples.

The estimates for power densities provided here are mostly upper limits. In most cases, power density will be quite a bit lower, due to all the components that I have not included the mass of because they are incredibly difficult to estimate. For example, the cooling system that maintains the superconducting generator at cryogenic temperatures, the mass of the structural support that holds the turbine in place, the mass of the recuperator that recirculates the turbine exhaust gases back into the inlet... and so on. The reason they are impossible to accurately estimate is because they depend on hundreds of factors for which there is no one number or answer. For example, the cooling system will be bigger if you expect higher heat load from going closer to the Sun, or the structural support will be lighter if you never intend to land the spacecraft...

PV cells in certain designs can reach frighteningly high power densities, sometimes surpassing that of a turbine. The choice between either system depends on factors such as the radiation environment, the expected mission duration, whether the craft is automated or manned (can perform repairs), whether you have access to rare metals or not, and so on. There's no clear-cut answer!

7 hours ago, YNM said:

Do you realize that heating things up is the least efficient of methods ? Also, how will the "gas" (working fluid) cool back down ? I don't think wire radiators.

'Heating things up' has an efficiency limited by the thermodynamic cycle. The working fluid is cooled back down with radiators. Wire radiators are radiators.

[YNM]

7 hours ago, MatterBeam said:

 

7 hours ago, MatterBeam said:

The working fluid is cooled back down with radiators. Wire radiators are radiators.

What, if I let the coolant in car radiator empty the engine will not melt down ? Or I can replace them with enough passive GPU heatsink ?

[wumpus]

2 hours ago, YNM said:

What, if I let the coolant in car radiator empty the engine will not melt down ? Or I can replace them with enough passive GPU heatsink ?

I strongly suspect that the engine will deliver heat to the heatsink/radiator via some fluid, and then the radiator will effectively be some sort of ultra-thin film of blackened magnesium/gold foil/blackened mylar.

I'm also deeply suspicious of the constant need to maximize temperature.  This is often done to maximize the efficiency of the energy source, but in the case of sunlight it is irrelevant (mirrors are cheap).  It is by no means clear that high temperatures are optimal for the radiator nor the mass of the engine.  Sacrificing max temperature for less mass (perhaps more Al or Be in the alloy) is likely to make more sense.  This was less obvious in part one where increasing the efficiency of a solar panel was unlikely to alter the mass (most of the mass is a support structure for a tiny layer of active silicon).

Don't underestimate the scale needed to make this thing work.  My understanding is that the radiators of the ISS have about 1/5 the surface area of the solar panels (and you would need plenty more radiators/W for a turbine), so you would be needing a vastly more powerful beast where radiators are essentially trivial (thin films with heat delivered via fluid).  This seems decades away.

[PB666]

On 12/27/2017 at 4:10 PM, MatterBeam said:

The entire blog? What would you suggest?

The whole blog needs to be rewritten.

[MatterBeam]

15 hours ago, YNM said:

What, if I let the coolant in car radiator empty the engine will not melt down ? Or I can replace them with enough passive GPU heatsink ?

In the vacuum of space, the only way to get rid of waste heat is through blackbody emissions from a hot surface. So, you provide enough area at a high enough temperature so that the blackbody emissions match your waste heat output.

12 hours ago, wumpus said:

I strongly suspect that the engine will deliver heat to the heatsink/radiator via some fluid, and then the radiator will effectively be some sort of ultra-thin film of blackened magnesium/gold foil/blackened mylar.

I'm also deeply suspicious of the constant need to maximize temperature.  This is often done to maximize the efficiency of the energy source, but in the case of sunlight it is irrelevant (mirrors are cheap).  It is by no means clear that high temperatures are optimal for the radiator nor the mass of the engine.  Sacrificing max temperature for less mass (perhaps more Al or Be in the alloy) is likely to make more sense.  This was less obvious in part one where increasing the efficiency of a solar panel was unlikely to alter the mass (most of the mass is a support structure for a tiny layer of active silicon).

Don't underestimate the scale needed to make this thing work.  My understanding is that the radiators of the ISS have about 1/5 the surface area of the solar panels (and you would need plenty more radiators/W for a turbine), so you would be needing a vastly more powerful beast where radiators are essentially trivial (thin films with heat delivered via fluid).  This seems decades away.

Increasing the temperature allows you to maximize the conversion efficiency of heat energy into mechanical energy. Radiators emit more heat when they operate at a higher temperature, so higher temperatures mean both better efficiency (less waste heat) and smaller radiators (less mass).

12 hours ago, PB666 said:

The whole blog needs to be rewritten.

You're restating yourself.

[PB666]

In fairness to the OP, I am not repeating myself, and I have had the same problem working at the journal. You tell someone nicely they need to rewrite and they don't get it.
OK so the second time you tell them. You need someone to review your site who is familiar with technical English. Your figures, not 1 but many are not readable. Some figures are unnecessarily oversized, there is alot of discontinuity in the writing style. its difficult to follow. Also are the photo's you use properly attributed to the authors, if not you maybe violating the fair use rule. You can make smaller low resolution version of those images that do not violate the fair use rule. The fair use states that if you copy a part of a work that is a significant part of that work depriving the creator of potential gain from his work then the use is unfair. An example given is a talk by George Bush in an interview, the author ask a number of questions, but the most significant question was only a small portion of the work, a paragraph. A third individual copied that paragraph without permission and attribution. The court found that the copied part was significantly representative of the original article and was in violation of the fair use rule. If you blog is on a site that gains money from advertising then be careful what you post that is attributable to others. Use wiki and follow the gnu.  Even with wikipedia, attribute.
 

16 hours ago, YNM said:

What, if I let the coolant in car radiator empty the engine will not melt down ? Or I can replace them with enough passive GPU heatsink ?

The problem with a radiator is that it works by keeping steam under pressure, its not  a closed system and therefore it would not work in space. The cap is generally set at about 1ATM which is ATM plus 1ATM. In space ATM is 0 so that a radiator would hold water in space but not once it was heated. An automobiles engine has holes in it that water passes, the water has a high heat capacity and so it picks up heat readily and carries it to the radiator.

So lets divide the space problem into three solvable problems. The first problem is that the radiator pressurization system is insufficient. Worse the radiator hoses in space would be constituitively under pressure and would age prematurely, the hoses would not radiate much heat and the exterior would also age faster. So that the plumbing on a radiator would need to be entirely replaced. The radiator cap would be replaced, it would need an internal overflow capture with a baffle attached to a pump that could evacuate the baffle, it would have a flow sensor on the overflow line that would shut off the engine but keep the water pump moving when the engine was off. (Not going to discuss the even greater problem with ICE in space)
Of course the cap would need to be sealed.

The second problem is the solvent to use. The problem with water is that it is volatile. You could replace water with a non-volatile liquid, like mineral oil but its thermal conductivity would fall. Waters structure is stable to heat, oil is not. So if oil is used you would need a reservoir and a feed system. Though as a heat conductor in the engine its will last longer, maybe years. Mineral oil is more viscous than water and so would need more pressure to move, because of its low heat capacity more would be required thus larger holes in the engine for the viscous lower heat capacity oil. The heat exchanger would have to be larger. The bore holes would need to be larger.

The radiator is not designed for space, a radiator is an air throughput convective heat transfer device. To put simple the radiator and fan are designed to take air around the heat exchanger move hot air away and bring cold air in. In this way heat is transferred. In space there is no air. A heat sink does the same thing. So now we have to replace the radiator with a much larger surface area that hopefully can pipe itself to the blackness of space.
If an engine is inside the car and a radiator is one hundreth the size, in space the radiator is outside the car and one hundred times the size. But you could put thermocouples between water pipes and heat sinks.

 

[Starman4308]

41 minutes ago, PB666 said:

In fairness to the OP, I am not repeating myself, and I have had the same problem working at the journal. You tell someone nicely they need to rewrite and they don't get it.

You... kind of did repeat yourself, giving no additional information. In this case, you weren't telling him nicely, you were saying "everything is terrible, rewrite" without the slightest indication of what was wrong.

The failure in communication here is not on MatterBeam's part, it is quite squarely on you. Ironically, usually you seem to go for large, incomprehensible, rambling walls of text, whereas here you went for a single-sentence dismissal of everything he wrote.

I'm also pretty sure downscaling images is not "fair use"; downscaled, upscaled, or edited, you still have to attribute the original source according to its license when you use it.

[PB666]

I gave him a chance to think about what he had written, may he would spot his errors, but after the second time I realized he wasn't going to try. I was only trying to help him.

I cannot give the material a fair critique because of the way its organized . in the world of manuscripts that a summary reject, you can't send it to referees if its so vague.

. . .he asked for critiques. You have to attribute images, the problem is the high resolution images are incontestably copyright infringement if they are not from a gnu resource or with given permission. You can get around this by using a low resolution image. If he seriously wanted me to pay attention to that birds nest of images, the first thing he would present is the clever strategies to radiate heat, because if he cant, all the other stuff is bunk. Another poster and I have already had the discussion in another thread, through two independent efforts to calculate the radiators were came to impossibly sized vessels. I like the idea of fusion, and fusion in space, conceptually I think its great, but the more I studied the problem, the more I came to realize the impossibilities of making it work without an even greater technological effort to improve extraction efficiencies. All he presents is old school  land-lubber stuff.

On the meta stuff.

Yes, walls of text, guilty, but my rambling walls of text are 1/20th the size of this blog and when I use images I generally attribute their source, and if there is math I do try to explain. The forum is not a blog page, its a chat room where people discuss ideas, sometime silly ones. A fixed web page is something that people attach to, like a FYI or sticky page in KSP, or a wike page on wikipedia.

The other problem is on the forum is that people frequently simplify an issue to the point that the simplification no longer has function. For example gravity around a black hole, the reason we have space-time is so that we can avoid presenting things like black perimeters in a flawed Newtonian context. In the last month I have probably corrected the gravity misconception 20 . . .30 times. If gravity simply accelerates matter, then it cannot lens light, so why do we use an obsolete terminology in the context of a relativistic phenomena. The other example is that rockets tip over without fairings, that blanket statement completely misrepresents the underlying physics. Both in game and out.

If you can stop folks from pushing misconceptions I will stop the walls of texts.

 

 

Edited December 30, 2017 by PB666

[MatterBeam]

21 hours ago, Starman4308 said:

You... kind of did repeat yourself, giving no additional information. In this case, you weren't telling him nicely, you were saying "everything is terrible, rewrite" without the slightest indication of what was wrong.

The failure in communication here is not on MatterBeam's part, it is quite squarely on you. Ironically, usually you seem to go for large, incomprehensible, rambling walls of text, whereas here you went for a single-sentence dismissal of everything he wrote.

I'm also pretty sure downscaling images is not "fair use"; downscaled, upscaled, or edited, you still have to attribute the original source according to its license when you use it.

I might have irked @PB666 due to previous interactions in other threads.
I don't down-scale images - in every case where they are lower resolution than expected, it is because I could only find a small image and had to fit it to the edge of the blog column.
I do comply with fair use rules here.

14 hours ago, PB666 said:

I gave him a chance to think about what he had written, may he would spot his errors, but after the second time I realized he wasn't going to try. I was only trying to help him.

I cannot give the material a fair critique because of the way its organized . in the world of manuscripts that a summary reject, you can't send it to referees if its so vague.

. . .he asked for critiques. You have to attribute images, the problem is the high resolution images are incontestably copyright infringement if they are not from a gnu resource or with given permission. You can get around this by using a low resolution image. If he seriously wanted me to pay attention to that birds nest of images, the first thing he would present is the clever strategies to radiate heat, because if he cant, all the other stuff is bunk. Another poster and I have already had the discussion in another thread, through two independent efforts to calculate the radiators were came to impossibly sized vessels. I like the idea of fusion, and fusion in space, conceptually I think its great, but the more I studied the problem, the more I came to realize the impossibilities of making it work without an even greater technological effort to improve extraction efficiencies. All he presents is old school  land-lubber stuff.

On the meta stuff.

Yes, walls of text, guilty, but my rambling walls of text are 1/20th the size of this blog and when I use images I generally attribute their source, and if there is math I do try to explain. The forum is not a blog page, its a chat room where people discuss ideas, sometime silly ones. A fixed web page is something that people attach to, like a FYI or sticky page in KSP, or a wike page on wikipedia.

The other problem is on the forum is that people frequently simplify an issue to the point that the simplification no longer has function. For example gravity around a black hole, the reason we have space-time is so that we can avoid presenting things like black perimeters in a flawed Newtonian context. In the last month I have probably corrected the gravity misconception 20 . . .30 times. If gravity simply accelerates matter, then it cannot lens light, so why do we use an obsolete terminology in the context of a relativistic phenomena. The other example is that rockets tip over without fairings, that blanket statement completely misrepresents the underlying physics. Both in game and out.

If you can stop folks from pushing misconceptions I will stop the walls of texts.

What is wrong with the way 'it' is organized? You are the only one to have ever remarked that organization of the content is a problem, since the blog's inception in February 2016, so I am curious.
There is zero mention of fusion here, so why is it a problem?
In which way closed-cycle solar-powered turbine 'old school land-lubber stuff'?
If you've got an issue with the differences between a forum and a blog... well, the content is presented both on the blog and this forum, so there's something for everyone.
Which misconceptions are being 'pushed' here, if you could point out one?

Edited December 31, 2017 by MatterBeam

[Starman4308]

20 minutes ago, MatterBeam said:

I don't down-scale images - in every case where they are lower resolution than expected, it is because I could only find a small image and had to fit it to the edge of the blog column.
I do comply with fair use rules here.

For downsizing, my interpretation of PB666's post was that he claimed downscaling an image made it fair use, which is incorrect.

When it comes to fair use, be careful, cite whenever possible the original work, and try to use public domain or permissive license images to avoid dealing with fair use in the first place.

Still, it's a blog post, not a manuscript that needs to be very careful about plagiarism issues. My understanding is that plagiarism is not a crime (though copyright infringement is), so private works don't strictly need to cite... though it's polite to cite anyways.

[IAmNotCreative_21]

Explosions are cool

[PB666]

10 hours ago, Starman4308 said:

For downsizing, my interpretation of PB666's post was that he claimed downscaling an image made it fair use, which is incorrect.

When it comes to fair use, be careful, cite whenever possible the original work, and try to use public domain or permissive license images to avoid dealing with fair use in the first place.

Still, it's a blog post, not a manuscript that needs to be very careful about plagiarism issues. My understanding is that plagiarism is not a crime (though copyright infringement is), so private works don't strictly need to cite... though it's polite to cite anyways.

If you get the resolution low enough, mostly the legal guys won't bother you if you cite the source. Again he has two problems. . . he does not cite his source and he is using high resolution, both combined can create problems for him.

https://www.rivaliq.com/blog/guide-copyright-fair-use-laws-online-images/

3. The amount and substantiality used: using only a small piece of the image, using only a small thumbnail/low-resolution version of the image

And secondarily if the writing on the image cannot be read, get rid of it. . . if you substantially alter an image its less of a fair use issue. If you only need to show part.

And you can find other sources. I should take bets with you folks who tell me I'm wrong.

In that spirit  . . . . .As I will restate for the last time, if he wants his presentation to be taken seriously he has alot of work to do, as it stands its entertainment so it does not fall under fair use guidelines.

Edited December 31, 2017 by PB666

[Starman4308]

36 minutes ago, PB666 said:

If you get the resolution low enough, mostly the legal guys won't bother you if you cite the source. Again he has two problems. . . he does not cite his source and he is using high resolution, both combined can create problems for him.

As originally stated, it was dangerously misleading. While downscaling might make courts more likely to agree it's fair use, it is not a get-out-of-jail-free card. You are still preparing a derivative work, a right given to the original copyright holder, and hoping courts would agree it's fair use.

I also find it improbable that a court would call it "entertainment". That was a qualitative judgement on your part, not related to the actual intent of the blog.

Overall, you seem to be applying standards based on your personal experience to a quite different type of media from a quite different type of author not used to your field's standards, and being incredibly high-handed and arrogant about it.

He is not somebody submitting to your journal. He is an enthusiast making a blog post. This is not a professional context where there are clearly established guidelines on content, where you could say "please re-read these guidelines and resubmit based on these guidelines"; it's an amateur blog post. Even in a professional context, your original "rewrite everything" posts would have been arrogant and uninformative; for an enthusiast blog post, they were simply unhelpful and insulting.

In an informal context such as this, it is crucial to give specific feedback, preferably couched in a constructive tone. Otherwise, you mostly just wind up insulting the author. Even in professional contexts, it's best to maintain a constructive tone, treating the submitter as a colleague instead of a misbehaving student.

And speaking of "treating him as a misbehaving student", you have failed to grasp the difference between privately telling a student "please resubmit" and publicly telling the world "I think your article is terrible and needs to be completely rewritten".

[PB666]

1 hour ago, Starman4308 said:

As originally stated, it was dangerously misleading. While downscaling might make courts more likely to agree it's fair use, it is not a get-out-of-jail-free card. You are still preparing a derivative work, a right given to the original copyright holder, and hoping courts would agree it's fair use.

I also find it improbable that a court would call it "entertainment". That was a qualitative judgement on your part, not related to the actual intent of the blog.

Overall, you seem to be applying standards based on your personal experience to a quite different type of media from a quite different type of author not used to your field's standards, and being incredibly high-handed and arrogant about it.

He is not somebody submitting to your journal. He is an enthusiast making a blog post. This is not a professional context where there are clearly established guidelines on content, where you could say "please re-read these guidelines and resubmit based on these guidelines"; it's an amateur blog post. Even in a professional context, your original "rewrite everything" posts would have been arrogant and uninformative; for an enthusiast blog post, they were simply unhelpful and insulting.

In an informal context such as this, it is crucial to give specific feedback, preferably couched in a constructive tone. Otherwise, you mostly just wind up insulting the author. Even in professional contexts, it's best to maintain a constructive tone, treating the submitter as a colleague instead of a misbehaving student.

And speaking of "treating him as a misbehaving student", you have failed to grasp the difference between privately telling a student "please resubmit" and publicly telling the world "I think your article is terrible and needs to be completely rewritten".

Again you are wrong, you are unfamiliar with publishing standards. So let me make it clear. If a scientist A publishes an image in a journal and scientist B takes the image an republishes several things might happen.
1. The first scientist may see the image or other work and may complain to the publisher (which, until about 6 months ago I would be part of)
2. The publisher and the first scientist may send a letter to the scientist or to the scientist's institutions. I have seen this happen multiple times
3. The publisher may tell the publishing journal to retract or create an erratum that then cites its source.
4. Take the publisher to court (almost never happens because they are happy in general to do 3).

This has to do with material that meets a certain standard as educational, his does not, (See 1). As entertainment the rules are a bit more stringent. So for instance if I am slowing slides at a conferance, in general that would be fair-use under just about all circumstances, but citations are desirous. Reasons: restricted audience, material unlikely to be further copied, assumption as educational.
Presenting the material in a moderated forum that is closed to the general public that discusses science, that would pretty well follow fair use, and I used to do that, but in a few instances I know people who have gotten in trouble, so its better to point to images and material than to copy and past them. Reasons: Moderators are like educators, there is a mechanism for removal, and its not generally accessed by everyone,

So here are situations that warning shots have been fired

An item that was in the prepublication state was taken and loaded into the file section of a scientific forum whose membership is open to the general public but files are not, a person working for the magazine was also a member of the forum, after complaining to the moderator the article was removed. This was in an educational forum, there was a non-distribution agreement for premium members who had early-access to the articles, the non-distribution agreement was violated. Different copyrights declarations have different stringencies go for the gnu. In general, unless the forum is very restrictive, don't copy and past large volumes of information. Copying and pasting a tweet is likely not a copyright infringment, the millions of encoded color information in a 1024 x 1024 image is. As a matter of fact after the incidence a letter was posted on the forum and sent to all members stating 'make sure you have permission to copy and distribute articles' . The institution I worked for did the same. Application here: since we don't have a citation for the source we don't know the copyright restrictions on the work.

Another incident, a popular book was published online. The educational institution had established a forum for which members could have their own web pages (a very bad idea in my opinion and I insisted that they remove all of my personal info from their pages - this type of behavior is now filtered- after the institutions information servers were hacked - any hacker now had all the information from a single institute to do a near complete identity theft - now they are paying "lifelock" a whole bunch of money to protect the identity of people who have not worked at the institute in two decades) . . . . anyway, a member of the institute who was familiar with page design inserted an electronic "warez" copy of the book on the information server. He sent emails to his friends. . at the institute on how to link the electronic copy. Those friends begin sending the information outside the institute. From the story as it was told to me it took about a week to for the publisher to get a certified letter to the institution telling them to "cease and desist". Yeah, they have the means to do THAT also.

Another incident(s). An author has published a work as a minor author with several corresponding authors in Journal X. The author leaves the lab and starts his own lab, continuing the type of  work in the first lab. Because his skills in English are not 'technical', he takes the published work, substitutes the differences between the first work with second work and submits the publication to a new journal. The article is summarily rejected because in its manuscript form more that 40% of its content is plagiarized. The author claims that since he was an author that he has the right to use the work, in correspondence with Journal X, the journal faxes him a copy of the original transfer of copyright that he signed. The second Journal (i.e. me) tells him that he can resubmit only after he removes the plagiarization and sends him a copy of fair-use rules.

So for example, if I saw Matter Beam publishing information that I knew was published in the journal that I worked for, I might ponder whether I would notify that journal of his blog page depending on the severity of the violation (for example, lack of citations, the number of images from a single source, the cutting and pasting of text information [including information embedded in images] , the resolution of the images). I pretty much know how they would respond, so all it takes is one pair of eyes that knows what they are looking at.

 

 

Edited December 31, 2017 by PB666

[KG3]

Just a question about the concept of putting a turbine into space.  What would having such a large rotating mass do for the stability of the spacecraft?  Wouldn't you need two counter rotating turbines?  Or maybe this system in intended to by landed and secured to a much larger body like a moon or asteroid?

[PB666]

7 minutes ago, KG3 said:

Just a question about the concept of putting a turbine into space.  What would having such a large rotating mass do for the stability of the spacecraft?  Wouldn't you need two counter rotating turbines?  Or maybe this system in intended to by landed and secured to a much larger body like a moon or asteroid?

Thats a problem but not the biggest one.

[KG3]

So what's the biggest problem, getting rid of waste heat?

[Green Baron]

41 minutes ago, PB666 said:

Again you are wrong, you are unfamiliar with publishing standards.

[+ wall of text snipped]

How do you know ? And do we really need this kind of effusions in here ?

This is not a scientific journal but a game forum. The standards that we have are absolutely ok though not always rigorously enforced ;-)  But your scenario should not go through the review process at all if was a real journal without anybody noticing. If it does, then there is something basically wrong with the process between submission, review, corrections, etc. until the real publishing goes, well, public.

If you would stick to your standards it'll be almost ok for me, but, with all respect, you have yourself have violated publishing and citation rules several times in here as well as kind of a history of claims that we could discuss in length & width :-) Last time with claiming Neandertals lived "at least 500kya" without a correct source. Which i then let go through because it was grossly off topic in the EHT thread.

Question is then: do you want to be measured yourself at these your standards ?

Better not, eh ?

[PB666]

2 hours ago, Green Baron said:

This is not a scientific journal but a game forum.

But the blog, which he referred to, is not, he asked for critiques. I didn't tell him that his artwork or text was $#!^. If you ask someone who has refereed 1000s of papers for a critique are you then going to say to that person, but don't use any of the skills you developed as a reviewer or journal? If you capsulize what a person on the editorial staff of  journal does . . . .its about content, logic, syntax, spelling, symbolism, peer-view (and their authorit), authority, imagery, referencing, style. We don't pick and choose what we want to review, we have to review all. I have even criticized K2, here when the symbolism was inadequate. I expect others to criticize me.  The core editors of a publication has to look at a work and decide somewhat subjectively, whether it will benefit the readers of the journal, that's really it in a nutshell. Publishing something puts it as a part of the published sphere forever. And when it is not, like the silencing of heretical gospils or the burning of the library at alexandria, society generally has regrets. Blogs used to be like a diary, more or less like mendels studies of peas,  but now they are closer to publications spirit of the process becomes zen of blogging.

There is a particular philosophy that I carry with regard to publication which I wish was shared by all scientist. Your best friend is your most hostile critic who is close to you, the closer they are to you and more critical they are, the more, ultimately, they will benefit your work as long, in most cases, that the work is not rejected. I have seen many a time when people clamp their mouths when they saw something they knew was in error. It generally does not benefit the authors, the folks who, in my younger days I clamped my mouth, are no longer in the field and left the field unexpectedly quickly. It was fortunate that my graduate work was in a lab with a very critical and vocal advisor that encouraged critique.

[YNM]

On 30/12/2017 at 9:34 AM, MatterBeam said:

In the vacuum of space, the only way to get rid of waste heat is through blackbody emissions from a hot surface. So, you provide enough area at a high enough temperature so that the blackbody emissions match your waste heat output.

Good luck with that, I'm definitely not cooling my spacecraft down with bunch of passive coolers tied by ropes. A car radiator would still be better.

On 30/12/2017 at 11:32 AM, PB666 said:

The problem with a radiator is that it works by keeping steam under pressure, its not  a closed system and therefore it would not work in space. The cap is generally set at about 1ATM which is ATM plus 1ATM. In space ATM is 0 so that a radiator would hold water in space but not once it was heated. An automobiles engine has holes in it that water passes, the water has a high heat capacity and so it picks up heat readily and carries it to the radiator.

The ISS has radiators with coolant you know. And that's not having any super hot parts.

 

I'm sorry, I know it's meant to be "tough SF", but my very bad sense tells there's a constant disregard to proper thermodynamics. IRL thermodynamics are very important - even to fields not really concerning with actually utilizing heating/cooling/advanced fluid mechanics - so only seeing the good sides and disregarding their bad sides is just soft "magic".

For instance, it kept being said that the turbines can run at very high efficiency. But the fact that it needs cooling means some of that energy is simply lost yet again. Very high efficiency engines don't have coolers - for instance, Stirling engine - but they're impractical indeed.

Just get ribbons of PVs or something.