Natural gas engines cost considerably less to operate than electric motors- nearly 30-40% less during on-peak and mid-peak hours due mainly to eliminating electric demand charges during those times. In order to maximize savings and provide high system reliability, hybrid systems with both engines and motors offer significant benefits. Such a system will allow the electric units to operate at night when electric demand is low and power is cheap, while the engines operate during the daytime when electricity is a premium. Not only will the facility achieve savings around the clock, there will be inherent fuel redundancy in pumping system.
Engines are inherently capable of varying their operating speed, which can yield part-load energy savings. An engine can be easily programmed to maintain a specific speed by automatically adding more combustion air and fuel (analogous to a car in cruise control that maintains a constant speed even when going uphill). In order for an electric motor to vary its speed, it must be equipped with an adjustable speed drive, which can add quite a bit to the cost of the motor system.
The ability to recover thermal energy produced by an engine can significantly enhance the economics of a project. Nearly 50% of the heat content of the input fuel can be recovered from the jacket cooling water and exhaust gas. That is like cutting the fuel price in half for that incremental load, not to mention the resulting increase in overall system efficiencies. One can recover close to 200F water off the engine jacket and higher temperature energy , 700F to 1000F from the exhaust. Certain industrial processes, boiler make-up water heating, absorption chilling, etc. are excellent candidates for heat recovery applications. Desiccant dehumidification systems that could use heat from the engine may be feasible in wastewater plants to reduce humidity and condensation.