Low Temperature Difference Stirling Engines & non-concentrating solar

Terraformer · 2024-02-11 11:05:46

Deserts are famous for their diurnal temperature swings, regularly dropping below the freezing point of water at night. Asphalt in direct sunlight at high latitudes can reach 50-60c, which suggests that such temperatures should easily be achievable in a hot desert using very cheap and simple unglazed flat plate collectors.

This opens up the possibility of a diurnal heat engine, that cools a heat sink during the night and heats a heat source during the day. Such an engine would have a Carnot efficiency of 18%. The actual efficiency would be significantly lower, but by how much? It is frustratingly hard to find figures for the fraction of the theoretical maximum efficiency that real world engines can achieve. Even more so for low temperature difference Stirling Engines. I have seen figures that suggest they can possibly achieve half of Carnot, but I have very little experience with heat engines. They're not my main area of interest or study.

But if they really can get half of Carnot, then a non-concentrating solar power plant in a desert might be able to convert 9% of received sunlight into usable energy. Considering conversion losses and other such things, maybe 5% conversion over all. Which is 4-5x lower than solar PV. But the system would be simpler to manufacture, provide power reliably, and we have quite a bit of desert to use. 5% of the sunlight on the Great Australian Desert still works out to 12.5W/m^2. Covering an area the size of Great Britain in that case would give 2.5TW of power for energy intensive industries such as aluminum smelting and glass production. Or synfuel. (I picked Australia on the assumption they'd be friendlier to Britain than other places with suitable deserts...)

tahanson43206 · 2024-02-11 11:22:04

For Terraformer re promising new topic!

Best wishes for success with this concept!

This post is available for an index to posts that you might care to highlight.

The 95% of energy your system does not capture is available to heat the planet, which is what it is doing now, except for the portion that bounces back to space.

(th)

SpaceNut · 2024-02-11 13:43:30

I did not know what the low temperature of an operating stirling would be but here is the bad news.

Low Temperature Stirling Engine

Low temperature Stirling engines have the advantage of being able to utilize lower-level heat from a variety of sources, such as waste heat from industrial processes, or from solar collectors. A low temperature Stirling engine can run using a hot source temperature between, say, 150 degrees Celsius and 300 degrees Celsius.

mostlyt it seems to be about the differential of the extreme that we desire.

25 kW Low-Temperature Stirling Engine for Heat Recovery, Solar, and Biomass Applications

Calliban · 2024-02-12 05:59:39

Intriguing idea. I looked into something similar a while back using a low pressure steam turbine. I wanted to examine if I could build a home solar thermodynamic plant using vacuum solar thermal flat plate panels. LP turbines used in powerplants operate with condenser pressure of 70mbar, which allows heat to be rejected at 30°C. In theory, a heat source with a temperature of 60-100°C, could generate steam at pressures <1 bar, which could drive an LP turbine and generate power.

The problem is that the pressure ratio across the turbine is also < 1 bar. That means a very large turbine is needed per unit of power. Not just the turbine, but also the boilers, steam dryers, condenser and solar collector. You need a lot of plant per unit power. This means reduced power density and higher capital cost. If efficiency is lower, then the plant is also moving a lot more working fluid per unit power. This means greater parasitic losses from friction and feed pump inefficiencies. There is also the problem of temperature drop across heat exchangers. For a heat exchanger to work, there has to a temperature difference between one side and the other. This degrades the already modest temperature difference between the hot and cold source, because the actual temperature difference that the heat engine works between, will always be smaller than the dT of the hot and cold source. The design of heat exchangers is always a tradeoff between capital cost and temperature drop.

On the plus side, lower temperature systems operate at reduced pressure. This does reduce build costs somewhat. Systems that operate under vacuum or at pressure <0.5 bar(g), can even use concrete pressure shells, which are vastly cheaper than chrome moly castings. So the concept is not hopeless. But engineering a system that can generate positive net energy given these problem is challenging. Another similar idea is a solar or geothermal assisted powerplant. In this concept, solar or geothermal power is used to preheat feedwater sent into a boiler that is fuelled by coal or biomass. The use of a supplemental source of heat does reduce the fuel consumption of the powerplant.

Last edited by Calliban (2024-02-12 06:05:35)

Calliban · 2024-02-12 06:37:50

Butane would be a good candidate working fluid for a low temperature thermodynamic cycle. Phase diagram below.
https://www.engineeringtoolbox.com/docs … iagram.png

At 0°C, vapour pressure is almost exactly 1 bar. At 50°C, it is about 6 bar. So a thermodynamic plant working between 0°C and 50°C, can achieve a pressure ratio of 6 across an LP turbine. And butane is quite dense, with a molar mass of 58. So the turbine can be reasonably compact, at least comparable to a standard LP turbine in a steam plant. As condenser pressure is 1 bar, the condenser can be made from concrete, with plastic piping. These pressures are low enough that we don't strictly need feed pumps either. We could engineer a system where the height of the condenser above the boilers is enough to feed them with condensate using hydrostatic pressure. That saves a lot of capital cost. With pressures this low, I wonder if the boilers, dryers and turbine, could be housed within blocks of precast concrete?

Low temperature systems like this have a lot of options for building thermal storage into the system. This solves a very big problem. Our cold source could be an underground ice store. The hot source, a tank of hot water or melted paraffin wax. Such an arrangement could store days or even weeks of energy.

Last edited by Calliban (2024-02-12 06:42:44)

Terraformer · 2024-02-12 14:29:09

Ha I was thinking butane when reading your first post. It seems ideal for a system using ice as the heat sink. You can put 330MJ into a tonne of ice before melting it. At 10% conversion efficiency that's... 9kWhr per tonne melted? Hmm. We'll need a fair amount. Unless we can tap the oceanic depths for heat (but then, pumping losses).

Edit: oh okay, at 6kWh per m^2 insolation and 10% efficiency, we'd need to melt 1/15th of a tonne for each square metre of panel every day. Less than I feared, there's plenty of room for the water. A hectare 1m deep would let us generate 90MWh under those conditions. 45 assuming 5% efficiency thereabouts. A 2MW system, if nighttime cooling is doable? Or more even, since night generation wouldn't be dumping heat to ice but to whatever we're refreezing it with.

Last edited by Terraformer (2024-02-12 16:52:04)

SpaceNut · 2024-02-12 19:00:34

Ethane as well as others (combinations of carbon, hydrogen plus other gasses) are being used with heat sources whether hot or cold with a great enough temperature between them.

New Mars Forums

Announcement

#1 2024-02-11 11:05:46

Low Temperature Difference Stirling Engines & non-concentrating solar

#2 2024-02-11 11:22:04

Re: Low Temperature Difference Stirling Engines & non-concentrating solar

#3 2024-02-11 13:43:30

Re: Low Temperature Difference Stirling Engines & non-concentrating solar

#4 2024-02-12 05:59:39

Re: Low Temperature Difference Stirling Engines & non-concentrating solar

#5 2024-02-12 06:37:50

Re: Low Temperature Difference Stirling Engines & non-concentrating solar

#6 2024-02-12 14:29:09

Re: Low Temperature Difference Stirling Engines & non-concentrating solar

#7 2024-02-12 19:00:34

Re: Low Temperature Difference Stirling Engines & non-concentrating solar

Board footer