Waste Heat to Power
There is a lot of interest in recovering waste heat. Combined Heat and Power (CHP) is at the heart of a large part of the distributed generation business. The heat can be used to heat water, provide process heat, and even cooling. Converting waste heat to electric power is getting attention as well.
On the bleeding edge, there are a number of efforts underway to do it without moving parts–solid state conversion (of course not limited to waste heat). There are programs, for example to put thermoelectric converters on the exhaust manifold of diesel trucks, with the goal of replacing the alternator. Thermoelectrics, thermionics, thermophotovoltaics — all are being pursued with renewed vigor, in the hope that new physics can overcome the longstanding problem of high cost and very low efficiency…a subject for another day.
Waste heat gets wasted only because it tends to be hard to use. A diesel engine converts about 1/3 of the fuel energy to useful work (electric power, in the case of a genset) — the rest goes off as waste heat in cooling water and exhaust — unless a cost-effective means can be found to use it, as in CHP.
Making more electric power with the waste heat is another matter. The age-old Rankine cycle, the basis of all steam power plants, can be made to work at lower temperatures by using something other than water as the working fluid ("refrigerant"), typically an organic compound, thus the term "organic rankine cycle" (ORC). In effect, this is a heat pump or refrigerator running backwards. Instead of using mechanical energy to create a temperature difference, mechanical energy is produced by a temperature difference.
The main challenge isn’t the theory, it’s the practical difficulty of doing it. Factors such as temperatures (inlet and outlet), flow rates, size and type of heat exchangers, type of expander, materials, controls, etc. must be considered in the trade-offs of cost, performance, reliability and longevity.
It’s a lot harder than it looks. Despite many attempts, and the obviousness of the basic idea, there are actually not very many commercial providers of such systems, particularly in smaller sizes which can operate effectively at lower waste heat temperatures.
UTC, for example, announced it’s new "PureCycle" 200 kW unit only last Fall. It requires inlet temperatures above 500 deg F.
Ormat, (ORA-NYSE) long established ORC maker for geothermal plants, is moving into the industrial waste heat market. They too need relatively high temperature, for units in the 250kw – MW range. They also sell small standalone ORC-based generators which burn a fuel as the external heat source.
In Europe, one can find Turboden (Italy), Triogen (Netherlands) and FreePower (UK). All require high temperature, with the possible exception of FreePower, who say they can operate as low as 230 deg F.
High temperature means industrial processes that put out high temperature waste heat. Ormat, for example, has a 1.5 MW showcase unit that takes air at 520 deg F from a cement plant in Germany.
The water jacket of the lowly diesel engine, however, can only be allowed to go to around 230 deg F (and the water must be returned no cooler than around 215 deg F). While such temperatures can be readily adapted to CHP uses, power conversion is more difficult.
Cooler Power, Inc, a startup in California, has successfully built units that work in this range. The engine’s cooling water is taken (before it goes to the engine’s own radiator), and is fed to a heat exchanger where it is heated further by the engine exhaust. In another heat exchanger, the hot water heats and vaporizes the organic working fluid, which then drives the expander which turns the generator. The expander is key. In principle, any compressor technology can work backwards to act as an expander: scroll, screw, turbine, or piston. All have been used at one time or another. Cooler Power initially used a scroll, but then developed its own proprietary modification to a commercially available screw compressor, as the heart of the system. They have a patent in final review stages covering the modification and use of the screw expander, as well as the control system and choice of working fluid.
Cooler Power has proprietary software to develop process flow diagrams to size and specify components or installations. The proprietary Program Logic Control (PLC) circuits are designed for optimal failsafe performance and contain algorithms that are protected from reverse engineering. Each of the key components (heat exchangers, expanders, condensers, generators) are designed to last 20+ years and come from one or more sub-sectors of the existing industrial equipment industry.
The system can be scaled to fit applications ranging from 50 kW – 1 MW. Installed costs are in the range of $1500-1800/kW. Depending on the sales price for power, payback can happen in 2 years or less. It’s important to emphasize that this is green power, which usually enjoys premium pricing. There is no fuel cost; operating costs are very low; and there are also (monetizable) environmental benefits.
A showcase 50 KW beta unit was installed in 1992 at the Newby Island landfill site in Milpitas, CA, on a 1 MW engine. A new 150 kW system will come on line in March. The company anticipates installing 10 units in 2005, with rapid growth thereafter based on already-established customer and marketing relationships, selling both systems and power. They raising an equity investment round currently, and welcome both investor and customer interest.
Ray Smith, COO
Cooler Power Inc,
Redwood City, CA