Do you use a generator that runs on diesel or gas to power your business due to frequent power outage from the grid? Are you running an air-conditioner with the grid power? Then you must look for waste heat recovery system to improve your energy efficiency and save your fuel cost. You can also use roof top solar hot water to supplement waste heat recovery. The savings may be real and you may be able to recover your investment in a short period and also contribute for the reduction of greenhouse emissions.
The diesel or gas engine converts only most 30% of fuel input in the form of thermal energy into mechanical energy to run your generator, and the balance heat is wasted in the form of greenhouse gas. You can recover this heat and increase the efficiency of the system. This means for the same amount of diesel used, you will get much higher output in the form of heating or cooling or in the form of additional electricity.
The exhaust temperature from a gas engine is about 420C.You can also recover extra heat from jacket cooling. Let us assume that you have a natural gas-fired engine to generate 100kw electricity for the premises. The efficiency of such spark ignited reciprocating gas engines are typically about 30%, which means a natural gas input of 1.145 mm Btu/hr. Let us assume the cost of piped natural gas at $10 per mm Btu; the fuel cost will be about $ 11.45/hr.
The exhaust heat from the engine will be about 801,500 Btu/hr; with waste heat recovery efficiency at 75%, the heat recovery will be 601,125 Btu/hr.You can air-condition premises with an area of 35-40 square meters using this recovered waste heat. If you use grid power at the rate of $0.10/kwhr, to run the air conditioning system for the above area, you will be spending about 30,000kwhrs of electricity per month, costing about $ 3000 per month. By installing an absorption chiller to air-condition your premises using engine exhaust heat, you will be saving about $36,000 per year towards air-conditioning. The air-conditioning system may cost about $130,000, and with the above savings you will be able to get a return on your investment in less than 3 years.
If you have a roof-top solar water heater then you can supplement it with your engine exhaust heat water so that the capacity of the air-conditioning can be increased. It is one of the best methods by which an energy efficiency of a fossil felled engine can be increased. If the capacity of the engine is much higher, there are other methods by which the efficiency can be increased.
For example, the hot water from the exhaust system can be used to generate some extra power using an ORC, organic Rankin cycle. It is similar to a steam turbine. An organic liquid with low boiling point will be evaporated into vapor by a low heat source such as hot water from engine exhaust, which runs a turbine, generating some extra power and condensing back into the liquid, and then the cycle continues. You will be able to generate an extra electricity of about 15-18% making the total electrical efficiency of the system to 45-50%, which is similar to a Fuel cell system, but at a much lower cost.
Heat recovery system with an absorption chilling and using low heat source to generate extra power using ORC, are best methods to improve energy efficiency of an existing system with little investment. The purpose of such integration is to increase the energy efficiency of the existing system, so that you will be getting more output of energy from the same input of fuel.