in Table 2. While this downgrading of the COP due to power plant inefficiency and transmission losses is unfortunate, the electrically driven heat pump is far more efficient than resistance heating, which would have an EFF. COP of .3. It is also better, during a large part of the heating season, than a gas or oil fired boiler which would have an efficiency between 75% and 80% with continuous operation at its maximum heating capacity.

The heat pump with engine drive has seen only very limited use in the past because of its high first cost, high level of maintenance and poor record of reliability of the engine. This picture has, however, changed considerably in the last few years. New materials development and higher rotating speeds with lower piston speeds (shorter stroke) have resulted in engines with longer lives, increased brake horsepower and lower cost (7). While these improvements have not brought the cost of buying and maintaining an engine driven heat pump down to the level of an electrically driven one, these costs are much more than offset by the reduced operating cost if use is made of the heat recovered from the jacket, manifold and exhaust gas of the engine. The smaller operating cost of the engine driven heat pump is due to its high EFF. COP and thus more efficient use of fuel. As an illustration, Table 3 gives the heat equivalent of fuel input per bhp output and the amount of heat recoverable for a particular naturally aspirated gas engine (8). The heat recovery system is an ebullient cooling type which takes advantage of the heat of vaporization of the jacket water by allowing steam to be generated in the jacket itself.

Using the information in this table and assuming that the COP of the engine driven heat pump is the same as the COP of the heat pump discussed above with electric drive **(See Table 2), we find the EFF. COP of the engine driven system as follows:

where use is made of the fact that 1 HP = 2544 Btu/hour. This results in the EFF. COPS shown in Table 4.

**

Actually the COP for the engine driven heat pump will be slightly higher because an electric motor is only 85 to 90 percent efficient.

A naturally aspirated gas engine driven heat pump will thus have an EFF. COP in the temperature range 60 F to -10 F, between 1.6 and .85 as opposed to an EFF. COP between 1.2 and .42 for an electrically driven heat pump. Put another way, at 45°F the electric motor driven heat pump will use almost 50% more fuel than the heat pump with gas engine drive for each unit of heat produced, while at -10°F this figure increases to 100% more fuel consumed. The above analysis and the results presented in Tables 2 through 4 indicate that an engine driven heat pump is the most effective device available today for heating from the standpoint of energy conservation. This analysis does not, however, take into account non-steady operation of either the engine driven or motor driven heat pump because the required information is not available. In addition, the discussion was limited to the direct use of the recovered heat from an engine driven heat pump for heating purposes. This recovered heat can also be used to drive an absorption heat pump to provide either heating or cooling. This will be discussed further in the section on the Absorption Refrigeration Cycle.

The electric driven or engine driven heat pump can utilize either outside or inside heat sources and sinks (9). The most commonly used outside source-sink is air which is universally available and free. This medium is used almost exclusively on residential installations for this reason. Water from well, lakes and rivers is also an excellent source-sink. Unfortunately the cost of drilling wells or owning a lake makes these source-sinks prohibitive to almost all residential and small commercial installations. In addition, extensive heating up and cooling down of lakes and rivers can have a detrimental effect on their ecology. The earth can also be used as a source-sink of heat although it has not been found in the past to be very practical and dependable. The use of solar collectors as a heat source for heat pumps is still in the research stage. The idea, however, has considerable appeal because of the heat pump's ability to absorb the solar heat at a relatively low collector temperature (9). In addition, operating at low collector temperatures reduces the heat losses of the collector and consequently increases its efficiency.

When the space inside the building is both a source and sink of heat, the heat pump acts as a heat transfer system. The transfer cycle involves taking heat from areas of the building where cooling is required and making it available where heating is needed. This is sometimes also referred to as bootstrap heating or heat reclaim (10). The transfer cycle has application in many office buildings where the interior areas require cooling all year long during occupied hours (10). This is due to the numerous sources of heat, such as lights, people, computers, cafeterias, elevators, transformers, motors, etc. (11). In addition, heat can be salvaged when needed from the building exhaust air, the lavatory exhaust air, and sewage system. This heat is then available for space heating, reheat for humidity control, preheating domestic hot water, etc.

The advantages of the transfer system are that heating and cooling are provided at the same time, the heat pump operates at a relatively high COP, and the wasteful bucking of mechanical cooling equipment and fossil fuel heating equipment against each other is eliminated. The disadvantage of the transfer system is that, unless some means of heat storage is provided, heat is only available when a cooling load exists. When more heat is being lost from the building than is being generated by internal sources, which may be the case when the building is unoccupied and/ or the outside temperature is low, an additional source of heat must be provided. This can be done by using the heat pump system to extract heat from an external source, such as the outside air, or by employing additional conventional heating equipment, such as resistance heaters or gas or oil fired boilers.

To illustrate this method of heat reclaim, let us consider some of the heat transfer cycles which are presently being used in many buildings. Figure 9 shows a water-to-water heat pump employing a double bundle condenser (12). The condenser contains two entirely separate water circuits; one going to the building heating coils and the other to a cooling tower. This allows part of the heat rejected by the compressor consisting of the cooling load plus the horsepower input, to be used in heating, while the remainder is rejected to the cooling tower without contaminating the house water circuits.