As mentioned earlier, a heat pump is a device that gets heat at a certain temperature and releases this heat at a higher temperature. When operated to provide heat (e.g. for space heating or water heating), the heat pump is said to operate in the heating mode; when operated to remove heat (e.g. for air-conditioning), it is said to operate in the cooling mode. In both cases, additional energy has to be provided to drive the pump. Overall, this operation becomes energetically attractive if the total energy output is greater than the energy used to drive the heat pump and economically attractive if the total life-cycle cost (including installation, maintenance and operating costs) is lower than for a competing device.
The common heat source for a heat pump is air, although water is also used in many applications. During the past decade ground or geothermal resources have received increasing attention to be used as a heat source, particularly in residential applications. From the sectoral utilization point of view, air is considered the most common distribution medium where the heat pump provides both heating and cooling. For heating only, air is also a common medium, except in those regions where many water distribution systems are installed in the residential sector. The energy needed to drive a heat pump is normally provided by electricity or fossil fuels, such as oil or gas.
The general characteristics of some typical commercially available heat pump systems are listed in Table 4.1 for the residential, commercial and industrial sectors. For the commercial sector, all the basic characteristics are similar to those in the residential sector except for the fuel drive. In the former sector, a greater variety of fuels can be used because of the larger-scale operation which suits fossil engine systems. In industry, large-scale uses also result in greater fuel flexibility and the heat source is usually waste hot water, steam or humid air. The type of heat sink will depend on the particular industrial process.
The heating and cooling of single and multi-family houses has become the most successful application of heat pumps thus far. A large variety of systems exists depending upon whether they are intended for both heating and cooling or only heating, the nature of the low temperature source and the medium distributing the heat (cold) to the building (air, water, etc.).
The heating-only heat pump is applicable to the residential sector in many countries where there is no air-conditioning load. Units can be installed separately or as add-on devices. While performance tends to be higher than for existing systems, the major difficulty is that the higher first-cost of the unit can be recovered only over the heating season, in contrast to heating and cooling units which operate throughout the year. As indicated earlier, the electric add-on heat pump is a system that can be used in conjunction with fossil fuel-fired furnaces or with central electric warm air furnaces.
For the residential sector, output requirements from a heat pump vary according to the use to which the output is applied, as indicated in Table 4.2. The requirements of a single family residence will range from 4 to 30 kW depending on the size, type and degree of insulation of the building. Multi-family building needs range from 20 kW for a two-family residence, to 400 kW for an apartment block, although non-central installations involve smaller size units. Depending on the size of the grid, district heating schemes can range from 400 kW for a localized application to 10 MW for a large-scale system. The output needs of the commercial sector range from 20 kW for shops and small offices to 1 MW for large commercial centers. A greater range, from 100 kW to 30 MW, is found in the industrial sector. The delivery temperature also varies with the requirements of a particular application. Table 4.3 summarizes the temperature requirements for a number of uses.
Heat pumps for residential heating and cooling can be classified into four main categories depending on their operational function:
• Heating-only heat pumps for space heating and/or water heating applications.
• Heating and cooling heat pumps for both space heating and cooling applications.
• Integrated heat pump systems for space heating, cooling, water heating and sometimes exhaust air heat recovery.
• Heat pump water heaters for water heating.
In residential applications room heat pumps can be reversible air-to-air heat pumps (ductless packaged or split type units). The heat pump can also be integrated in a forced-air duct system or a hydronic heat distribution system with floor heating or radiators (central system).
They often use air from the immediate surroundings as heat source, but can also be exhaust-air heat pumps, or desuperheaters on air-to-air and water-to-air heat pumps. Heat pumps can be both monovalent and bivalent, where monovalent heat pumps meet the annual heating and cooling demand alone, while bivalent heat pumps are sized for 20-60% of the maximum heat load and meet around 50–95% of the annual heating demand. The peak load is met by an auxiliary heating system, often a gas or oil boiler. In larger buildings the heat pump may be used in tandem with a cogeneration system.
In commercial/institutional buildings the heat pump system can be a central installation connected to an air duct or hydronic system, or a multi-zone system where multiple heat pump units are placed in different zones of the building to provide individual space conditioning. Efficient in large buildings is the water-loop heat pump system, which involves a closed water loop with multiple heat pumps linked to the loop to provide heating and cooling, with a cooling tower and auxiliary heat source as backup.
In residential, commercial and institutional buildings, recently, there is an increasing interest in room type controlled heat pumps (Figure 4.1). In addition to some benefits such as greater comfort, reduced noise and reduced energy use, some features of this type of system are:
• preventing operation when connection is made to the wrong supply voltage or if the wiring is incorrect,
• preventing overheating of the compressor, fan motor and power transistor,
• detecting refrigerant undercharge and evaporator freeze-up, and
• maintaining the pressure balance by controlling the on/off switching cycle of the compressor.