1903 six HP steam engine powers modern off grid power system

[buenijo]

One way to get high efficiency in a piston steam engine is to use a compounded piston steam engine with steam reheat and heat regeneration. Three cylinders or more is preferable. There are many ways to go about this, but all have the same basic goal: expand high pressure steam through the engine while reheating the steam before it expands into the next cylinder - this boosts temperature (and therefore volume and/or pressure) to increase engine output - then regenerate the excess heat back into the system. Thermodynamically the ideal state of affairs is heating the steam as it moves through the engine such that the steam temperature is never allowed to drop below max, fully expanding the steam down to condenser pressure, then sending the very high temperature but low pressure steam through an air preheater that is able to cool the steam down to saturation, sending the heated air to the furnace, then sending the low pressure saturated steam to the condenser. This is basically what modern steam power plants do to approach 50% net thermal efficiency. There is no physical reason why a piston engine cannot be devised to see similar performance, but it would be a nightmare to actually build something like this. A two cylinder compounded steam engine can see a part load efficiency equal to the peak efficiency of a very good small wood gas engine system (about 20%) while using steam at 500 psig and 600F if it were a good engine and made good use of this strategy. It could have all the advantages of steam power w/o the disadvantages of a wood gas engine system (quiet, much wider range of fuel sources, no fuel gas filtering and therefore no fear of fouling the engine with tar, slow moving and long lasting, a flatter efficiency profile, and all heat available at the condenser for ease and efficiency in cogeneration). Still, this option is not practical without access to the necessary hardware. For this reason biomass gasification for fueling internal combustion engines is the practical alternative for making use of biomass for cogeneration.

REFRIGERANT BOTTOMING CYCLE: A strategy that has been used in piston steam engine systems of the past was to transfer the heat in the system to a refrigerant part way through the cycle. This boosts the average pressure in the engine. Below a certain steam pressure, the higher friction in a piston engine can lead to diminishing returns. This is why small piston steam engines generally do not expand the steam below a certain value (generally keeping steam pressure well above atmospheric while in the cylinder). Well, this corresponds to a temperature on the order of 250F or higher. What can be done (and has been done) is to use the steam condenser to heat and vaporize a refrigerant under pressure. This high pressure refrigerant can then be used to drive another piston. This keeps cylinder pressures very high to minimize friction losses, and allows for extending engine operation to much lower temperatures. I am aware of this strategy applied to a large stationary piston steam engine power plant that increased overall efficiency by about 50%. Another strategy that was used on the Titanic was to put a low pressure turbine on the exhaust of their compounded piston engines. Unfortunately, small turbines are not generally efficient, especially at low pressure. So, a low power (small) system could use the former strategy to boost efficiency. If anyone is interested to try this, I recommend using a scroll automotive a/c compressor as the refrigerant expander. Remove the discharge reed valve and admit pressurized refrigerant through the unit backwards. This approach would be far simpler than alternatives largely because the compressor is already designed to contain the refrigerant. Also, I have referenced studies that indicate these compressors operate as expanders with reasonably high efficiency.

In my opinion, a better strategy to increase the efficiency of very small systems is to make full use of the heat from the system for other applications (i.e. space heating, water heating, water pasteurization, water distillation, absorption/adsorption cooling, biomass fuel drying, etc.). For this reason, perhaps the most rational design would be the simplest configuration that achieves good performance. In my opinion, the single-acting uniflow that admits steam with a bash valve or small poppet valve is the best candidate.