Back in the mid-70's I worked on a project for NASA to develop residential solar powered air conditioning. This was back when NASA was doing development work for DOE. The contract team I was working with (Honeywell, Lenox, Barber-Nichols) developed a 3 hp turbine system that used R-113 for the working fluid. We built about 8 demonstration units that were installed in various government locations around the country. We also built several 25 ton water chillers. The residential system used a small turbine that was geared down 60:1 to 1800 rpm to turn a Ford automotive compressor running an R-12 cycle. The system actually worked quite well, plus it generated electricity when there was not a big cooling demand.
The purpose of the project was to use "off the shelf" components to get the total cost from the aero-space realm down to something the average consumer could justify. The cost part was never successful. There are just too many heat exchangers, controls, gearboxes and and lube systems to get to a viable cost. Another contractor group led by GE used R-12 in both the power cycle and the refrigeration cycle. They used a piston engine instead of a turbine The engine looked very much like a refrigeration compressor. The turbine, of course, ended up being more efficient, but the GE system worked as well. Note: both R-113 and R-12 are being phased out because of their ozone damaging issues, but there are many other fluids that can be used.
The selection of the working fluid is based on what pressure you can stand to operate, whether you are using piston expansion or turbine expansion, and other considerations like corrosion, lubrication, flammability, health safety, etc. The system has to be evacuated (as in a refrigeration system) to remove all air and water vapor before the working fluid is added. Residual oxygen in the system is always a problem, as is any remaining water vapor - both lead to corrosion problems. The system has to be air tight (like your central air system) to keep the working fluid in and the air out. A high quality shaft seal is required on the gearbox or engine to keep the working fluid in the system. The early refrigeration pioneers used nasty fluids like ammonia, ethyl ether, and hydrogen sulfide before the days of the halogen (Freon type) fluids. There are a number of newer halogen fluids that have been developed to replace the older, ozone depleting fluids.
A couple of things to keep in mind with a closed Rankin cycle (these are the cold blanket items):
- A heat engine needs a hot sink and a cold sink to run. The cold sink is usually the problem. Unless you have a cool river or stream, you are stuck rejecting heat to the atmosphere. Cooling towers and evaporative condensers are two ways to get the cold sink down to about the ambient wet bulb temp. In the SW states, the wet bulb temps tend to be lower and you can get a lower sink temp in Arizona and places like that. Otherwise the lowest temp you can usually come up with is the ambient dry bulb temp - which is usually in the 80 - 100 deg F range in the summer.
- The laws of thermodynamics dictate that the efficiency of a heat engine has to be less than the theoretical Carnot cycle operating at the same temperatures. These are fancy words that say if you only have 100 or 200 deg F between the hot sink and the cold sink, you will be well under 18% efficiency, probably in the 8 - 10% range. This means that to come up with even one horsepower, you have to process a LOT of heat. Makes for big heat exchangers (boilers, condensers, solar collectors, etc.) This is where the high cost comes in.
- To give you a rough idea, it takes about 500 square feet of solar collector operating at 180 deg F and a condensing temperature of 80 F to come up with 3 hp on the shaft (mid-day with clear skies).
Another option is to skip the Rankin cycle and use a Stirling cycle which uses air (or another gas) as the working fluid. There have been high hopes for this cycle ever since Robert Stirling proposed in back in 1816. The Stirling cycle can theoretically more closely approach the Carnot efficiency limit than the Rankin cycle. The problem is that it takes a lot of area to transfer heat from a solid to air. That's why the radiator in you car and the coils on your home air conditioner are filled with fins. There are kits for small Stirling engines that run off the heat of a cup of coffee or even the heat from your hand, but they only develop enough power to overcome their own internal friction. Just like the Rankin cycle, with only 200 deg F or so between the hot and cold sink, it takes huge heat exchangers to develop any amount of horsepower.
There were a number of Stirling engine builders in the late 19th century that supplied engines for pumping water on the farm. You can still see these running at threshing demonstrations and other museum venues. These engines burned wood or coal and developed about 1/2 hp. The wood fire provided a hot sink temp of 500 deg F or so, and heat was rejected to the ambient air. A typically 1/2 hp machine is about the size of a 5-drawer file cabinet.
Sorry for the long dissertation. Guess I got on a roll.
Its a long answer for why you don't see any engines running between low temperature differences.
Its tough to compete with the power companies - they have 1000 deg F or more between their hot sink and cold sink.
Keep at it.
Terry S.
