Air Source Heat Pump for Emergency Generators

Emergency backup generators are designed to keep critical systems operating in the event of a power emergency. These units are found in most large commercial and industrial buildings as well as other facilities that have a critical need for electric power such as data centers, hospitals, and communications centers. Emergency generators are generally internal combustion diesel engines and are normally kept hot and ready to run in the event of a loss of utility-provided electric power. The required heating energy is normally provided by electric resistance immersion heaters (generally two heaters per engine, with a total power draw of 5,000 Watt (W) or more). These heaters are used to heat the engine block and coolant that circulates through the engine’s water jacket. The warmed coolant then gives up its heat to the engine block proper as it circulates, keeping the engine at temperatures normally found to be between 90°F and 110°F. Electric resistance heating converts electric energy to heat energy with a coefficient of performance (COP) of unity (1). COP is the ratio of output heat over the input energy or electricity. When it leaves the immersion heater to circulate through the engine the temperature of the water is often above 150°F. The circulation is by convection resulting from the rising of the heated coolant. As a result, the temperatures at the top of the engine block are generally very hot (as much as 120°F). As the liquid cools, it "falls" slowly to the lower reaches of the water jacket. A significant amount of unwanted heat is transferred due to the temperature differential of the hot upper part of the engine to the surrounding environment; that heat is lost. Significantly less heat loss is possible if the engine is uniformly kept at the desired temperature. A technology is emerging that provides the required heat energy by the use of an air source heat pump (ASHP) and circulates that heat energy through the engine with the use of a small (100W) pump, keeping the engine at a relatively uniform temperature. ASHP can have a COP as high as 4.0, depending on the heat quality of the air source. Three sites and a total of four emergency generators were monitored to quantify the energy savings potential of this technology. The results indicate that the immersion heaters run a very large percentage of the time; one heater appeared to run almost continuously, only shutting down when the monthly performance test was performed. The heat pumps consume less power and operate fewer hours when compared to the resistance heaters. These two factors, combined with the reduction in heat loss described above, provide considerable energy savings. One limiting factor is that the immersion heaters still need to operate when the temperature dips below approximately 50°F. When temperatures are below 40°F the heat pump ceases to work and needs to be replaced by immersion heaters in order to provide engine coolant heating.