Solar Steam Power System

http://reneweconomy.com.au/2013/return-of-the-steam-engine-cheap-storage-for-solar-34662
http://www.terrajoulecorp.com/unexpected-technology/how-it-works/
http://www.terrajoulecorp.com/unexpected-technology/what-it-is-not/

I've been aware of this for a while. I like the idea. In my opinion, it's not suitable for micro scale CHP, but I think it has promise for medium scale installations - and this seems to be the target market.

The way I understand this system based on what I read, and extrapolating from my knowledge of steam power, is that a two cylinder compounded engine is used. These systems provide steam at high pressure to a small cylinder where the steam expands, then exhausts the lower pressure steam to a larger cylinder for additional expansion before finally exhausting to the condenser. The system here uses Skinner unaflow engines that were manufactured as late as the 1940's. These engines had a reputation for high efficiency, and extreme reliability. During the day when the solar concentrators are generating high pressure steam, then the steam is sent to the high pressure cylinder. The larger low pressure cylinder is not used when additional energy storage is desired. Rather, the steam is exhausted to a large insulated steel pressure vessel filled with water. The steam exhaust raises the temperature and pressure of the water. When sufficient energy storage is achieved, or whenever higher power is desired, then the low pressure cylinder may be used in addition to the high pressure cylinder. When solar is not available (such as at night), then high pressure steam is not available, and the system draws low pressure steam from the pressure vessel by flashing water to steam to drive the low pressure cylinder. Turns out that there is negligible loss of efficiency by going this route. That is, while the engine is most efficient when full expansion is achieved with both cylinders, whenever only the high pressure cylinder is used, then the energy that would otherwise be used to drive the low pressure cylinder is stored in the pressure vessel for later use. Since the efficiency of these engines approach 20%, the overall efficiency of this system can be quite high with good concentrators. More important, the system achieves 24 hour power generation from solar without batteries, and the storage system is the most cost effective I've yet seen. I've argued before several times that when the "fuel" is free (as in solar), then "efficiency" is a four letter word: COST. For medium scale distributed solar power, this solution looks great to me.

The discussion on the web site about the benefits of piston steam engines in lower power ranges is spot on. Turbines are great for high power and constant output. Below a certain power that is quite high (on the order of one megawatt), the piston steam engine is more efficient, and the efficiency of piston steam engines vary little as the power is varied. So, the humble piston steam engine is ideal for more modest scale distributed energy. While this particular configuration is probably not good for micro scale systems, I take the position that the piston steam engine in other configurations (particularly with biomass fuel and extensive cogeneration) is also ideal in the micro scale.

See the comments by the CEO of the company in the COMMENTS section of the page:

http://www.greentechmedia.com/articles/read/Terrajoule-Unstealths-Distributed-Power-via-Solar-And-Energy-Storage

Turns out that the steam pressures and temperatures achieved with this system, along with using irrigation water pumped by the system to cool the condenser, will allow for good efficiency. The CEO claims 30% cycle efficiency with 70% concentrator efficiency, and this would allow for 20% overall conversion of solar energy to work. This would be outstanding. However, I am skeptical about the efficiency claim. Their system does not allow for steam reheat or heat regeneration as I understand the configuration. If steam temperature is limited to 600F as the claim, and there is no reheat, then the best efficiency this system would see is the low 20's%. When the concentrator efficiency is considered, then the figure drops to about 15% net conversion of solar energy to work. Now, this is still excellent. More important, the efficiency of the system is not important - it's all about COST. In my opinion, this is where this system shines. I think it's brilliant.

I was thinking on the merits of using this basic idea for a micro scale application. If we consider a 1000 gallon insulated propane tank (just a round figure), then this will store about 800 gallons of water. At elevated temperature, this is roughly 6000 pounds of water. Let's consider taking the pressure to 250 psig which corresponds to about 406F. Note that a lower pressure might be used, but I understand these propane tanks to be rated at even higher pressures. Now, let's assume we use the system to supply steam to drive an efficient piston steam engine, and let's consider the tank depleted once it cannot supply steam at a pressure greater than 50 psi. This corresponds to about 281F. Therefore, a fully charged tank would store about 750,000 btu of heat energy. A very small reasonably efficient uniflow steam engine exhausting to a condenser can achieve 10% thermal efficiency here. If we consider alternator efficiency at 80%, and inverter at 85%, then it's theoretically possible to generate about 15 KWh of electricity using this energy storage scheme. BTW, a 1000 gallon tank is roughly 4 feet in diameter and 10 feet long.

If we consider concentrators at 50% efficiency, then a region that sees solar insolation of 6 KWh per square meter per day would capture 3 KWh of heat (roughly 10,000 btu) per 10 square feet per day. So, charging the tank would take a concentrator roughly 750 square feet.

It's more interesting to consider more efficient ways to use this heat than power generation. Note that the steam could also be used to power an adsorption cooling cycle at high efficiency since the temperature is so high. It takes about 1.2 KWh of electricity to power a one ton a/c unit for one hour, and this would consume about 60,000 btu of heat energy stored in this system. However, if the heat were used directly, then then the same cooling could be done with about 18,000 btu of heat. Water distillation could be done if desired using the efficient staging procedure I described in another post (https://permies.com/t/31436/energy/Efficient-Water-Distillation-Staging). In fact, the steam exhausted from the final stage could still be at elevated pressure to provide air conditioning efficiently as well. So, the same heat could be cascaded in stages to provide water distillation AND air conditioning. Imagine recycling all the water in the home with distillation AND staying nice and cool in the high desert - with only solar energy! Electricity could be limited to basic lighting and other low power items, and a modest photovoltaic array and modest battery system could be used here. So, basically, I'm suggesting that solar thermal energy storage might be used in tandem with photovoltaics to leverage solar energy more effectively than PV alone.

NOTE: I am not considering the practicality or safety of this configuration.

Nice find Marcos! This system looks very promising, for starters distributed solar thermal systems of that scale will prove to be more efficient overall (once line losses and other complexities are factored in), the choice of a steam engine at that scale is also wise, in fact i'd argue that the steam engine is the only engine of choice for solar thermal since temperatures above 600 deg F become prohibitively expensive to generate and increase overall costs of solar collector. As you mention steam turbines only shine at high steady state power outputs, to which i'd also add high temps preferably of 1000 deg F +- 10 deg to the list. Part of the reason steam turbines evolved as the dominant heat engine for power generation was their ability to operate at higher temps and therefore higher efficiencies since lubrication was a non issue whereas there piston counterpart suffered above 600 deg F.

I think your ideas of using this concentrated energy for cooling and water distillation are a fascinating potential for hot areas with high cooling loads and high solar insolation. The system would act as a very effective electrical energy use displacer and would reduce electrical needs to some LEDs and and communication/power electronics at which point a very small PV system could cover it. One the best things about the system is that you could almost close the loop on a home water cycle. Since this system would likely be targeted for the off grider in the southwest my next question would be how to best integrate it into an aquaponic system?

Nick Raaum wrote:Nice find Marcos! This system looks very promising, for starters distributed solar thermal systems of that scale will prove to be more efficient overall (once line losses and other complexities are factored in), the choice of a steam engine at that scale is also wise, in fact i'd argue that the steam engine is the only engine of choice for solar thermal since temperatures above 600 deg F become prohibitively expensive to generate and increase overall costs of solar collector. As you mention steam turbines only shine at high steady state power outputs, to which i'd also add high temps preferably of 1000 deg F +- 10 deg to the list. Part of the reason steam turbines evolved as the dominant heat engine for power generation was their ability to operate at higher temps and therefore higher efficiencies since lubrication was a non issue whereas there piston counterpart suffered above 600 deg F.

There are some interesting modern piston steam engines being developed by a small company (Cyclone Power Technologies). They've been able to do away with oil lubrication which allows them to take steam temperatures a lot higher. I can't say anything about the long term viability of what they're doing, but I do have some basic knowledge of how their systems are configured. I consider their work as a window to seeing what is possible with small scale steam power. You may be interested to know that the peak steam temperatures their engines achieve are approaching 1500F, and they're talking about approaching 2000F. This is done by using a high compression uniflow piston system where the residual steam in each cylinder is recompressed into a tube placed in the hottest part of the furnace. The furnace for the system is mounted on top of the cylinders which are in a radial configuration. Each cylinder has an associated fuel injector in the furnace. A low volume of low pressure air is used to carry the fuel through the injectors where it ignites, then mixes with a high volume of air preheated by the furnace exhaust gases. The reheat tubes are placed near the injectors where the temperature is highest. The internal volume of the tubes are very small, so they can withstand the high pressures. Furthermore, the length of the tubes are tuned to set up a standing wave of superheated steam during compression to try and optimize heat transfer at certain engine speeds that might be used more often (like a constant speed of a genset for example). One small engine of theirs rated at 18 hp weighs under 100 lbs complete (including steam generator, furnace, condenser, etc.), and was tested at 31% overall thermal efficiency (including all losses like thermal losses from the furnace, furnace air blower, and feed pump losses. The engine output during the test was about 5 hp. Steam generator was maintained at 1000 psi and 1000F. This efficiency is on par with good small Diesel gensets. A difference with this engine beyond the more compact and lightweight design includes highest efficiency at part load where stationary gensets spend most of their time, and an efficiency profile that varies little over the power range. So, efficiency is high at low outputs and high outputs. By contrast, the efficiency of internal combustion gensets fall off the scale at low outputs (especially gas engines). Also, it's quiet, and with clean emissions (full combustion with virtually no NOx). The condenser may be air cooled or water cooled, but in either case the heat available at the condenser may be used directly in heating applications. The furnace exhaust temperature is about 350F. The company is also working on an automotive system that maintains a steam generator at 3500 psi and 1200F. That system is projected to show 35% net thermal efficiency. Weight of that system is 350 pounds for 100 hp. Note that the torque profile of a piston steam engine matches the need of an automobile, so there is no transmission required - and a 100 hp steam engine would perform as a much more powerful gas engine.

The company is also working on a 5 hp engine designed to be heated by a parabolic solar concentrator. That engine is designed also for 30% thermal efficiency, and weighs 25 pounds complete.

It's an uphill battle, but I am confident that piston steam engines will return. I don't know when or in what form, but we'll see them again.

Video describing the Terrajoule system. The simplicity and effectiveness of this approach is astounding. I love the irony of finding such an effective distributed solar power system with existing mass produced components and old piston steam engine technology. Consider that the more complicated systems are generally the least stable. I believe this approach will catch on.

Marcos Buenijo wrote:Turns out that the steam pressures and temperatures achieved with this system, along with using irrigation water pumped by the system to cool the condenser, will allow for good efficiency. The CEO claims 30% cycle efficiency with 70% concentrator efficiency, and this would allow for 20% overall conversion of solar energy to work. This would be outstanding. However, I am skeptical about the efficiency claim. Their system does not allow for steam reheat or heat regeneration as I understand the configuration. If steam temperature is limited to 600F as the claim, and there is no reheat, then the best efficiency this system would see is the low 20's%. When the concentrator efficiency is considered, then the figure drops to about 15% net conversion of solar energy to work. Now, this is still excellent. More important, the efficiency of the system is not important - it's all about COST. In my opinion, this is where this system shines. I think it's brilliant.

I'm gonna have to reconsider my position here. It turns out that large compounded piston steam engines have shown very high efficiencies in the past. The losses are greatly lessened with large expanders. Croft in "Steam Engine Principles and Practice" shows several large compounded piston steam engines using saturated steam showing well over 60% of Carnot efficiency. A very good compounded uniflow expander like the Skinner design should be better than a counterflow compound. So, I think this system can pull off 30% efficiency with steam at 600F and water cooled condenser down to 100F saturation temperature. Overall efficiency in electricity generation for this system could approach 20% - and that would be AC power.