COGENERATION
When an internal combustion engine is used to generate electricity, a large part of the energy supplied to the engine in the form of fuel emerges as heat in the exhaust from the engine or is dumped into the atmosphere by engine cooling systems. If this heat can be captured it can be utilized for space heating or for heating water, potentially making the energy usage much more efficient.
The efficiency of piston engine–based power generation varies from 25% for small engines to close to 50% for the very largest engines. This means that between 50% and 75% of the fuel energy actually emerges as waste heat. There are four primary sources of waste heat in an internal combustion engine: engine exhaust, engine case cooling water, lubrication oil cooling water, and, where one is fitted, turbocharger cooling.4 Each of these can be used as a source of heat in a reciprocating engine cogeneration system.
The exhaust gas contains up to one-third of the fuel energy and 30–50% of the total waste heat from the engine. Exhaust heat is not normally captured but it is straightforward to fit a heat-recovery system to the exhaust of an engine if the heat is required. The exhaust temperature is typically between 370 oC and 540 oC. This is sufficiently high that it can be used to generate medium-pressure steam if required. Otherwise, it can be used to generate hot water.
The main engine case cooling system can capture up to 30% of the total energy input. Cooling water exits the cooling system at up to 95 oC.5 In a cogeneration system this will be passed through a heat exchanger to provide a source of hot water. Engine oil and turbocharger cooling systems will provide additional energy that can also be used to supply hot water.
If all the heat from the exhaust and cooling systems of an engine is exploited, around 70–80% of the fuel energy can be used. However, this can generally only be fully exploited when there is a need for hot water. Engine exhaust gases have also been used directly for drying in some situations.
Since cooling systems are fitted to internal combustion engines whether the waste heat is exploited or not, the use of these systems in combined heat and power applications offers a logical extension of their application. Cogeneration systems based on small engines can provide power, space heating, and hot water to homes and commercial offices, while large engines can produce power and low-grade process heat for small industrial operations. The economics of these systems can be quite favorable where there is a use for the waste heat. As a con- sequence, the cogeneration market, particularly for small systems, is buoyant and is likely to become more so if fuel costs continue to rise.