Heat pumps in residential and commercial buildings
Heat pumps for heating and cooling buildings can be divided into four main categories depending on their operational function:
Heating-only heat pumps, providing space heating and/or water heating.
Heating and cooling heat pumps, providing both space heating and cooling.
The most common type is the reversible air-to-air heat pump, which either operates in heating or cooling mode. Large heat pumps in commercial/institutional buildings use water loops (hydronic) for heat and cold distribution, so they can provide heating and cooling simultaneously.
Integrated heat pump systems, providing space heating, cooling, water heating and sometimes exhaust air heat recovery.
Water heating can be by desuperheating only, or by desuperheating and condenser heating. The latter permits water heating when no space heating or cooling is required.
Heat pump water heaters, fully dedicated to water heating.
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 (in a European residence). 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 (CHP).
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).
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.
The different heat sources that can be used for heat pumps in residential and commercial buildings are described in the section Heat sources. The next paragraph describes the types of heat and cold distribution systems that can be used in buildings.
Heat and cold distribution systems
Air is the most common distribution medium in the mature heat pump markets of Japan and the United States. The air is either passed directly into a room by the space-conditioning unit, or distributed through a forced-air ducted system. The output temperature of an air distribution system is usually in the range of 30-50 C.
Water distribution systems (hydronic systems) are predominantly used in Europe, Canada and the north eastern part of the United States. Conventional radiator systems require high distribution temperatures, typically 60-90 C. Today’s low temperature radiators and convectors are designed for a maximum operating temperature of 45-55 C, while 30-45 C is typical for floor heating systems. Table 1 summarises typical temperature requirements for various heat and cold distribution systems.
|Table 1: Typical delivery temperatures for various heat and cold distribution systems.|
|Application||Supply temperature range ( C)|
|Air distribution||Air heating||30 – 50|
|Floor heating; low temperature (modern)||30 – 45|
|Hydronic systems||radiators||45 – 55|
|High temperature (conventional) radiators||60 – 90|
|District heating – hot water||70 – 100|
|District heating||District heating – hot water/steam||100 – 180|
|Cooled air||10 – 15|
|Space cooling||Chilled water||5 – 15|
|District cooling||5 – 8|
Because a heat pump operates most effectively when the temperature difference between the heat source and heat sink (distribution system) is small, the heat distribution temperature for space heating heat pumps should be kept as low as possible during the heating season.
Table 2 shows typical COPs for a water-to-water heat pump operating in various heat distribution systems. The temperature of the heat source is 5 C, and the heat pump Carnot efficiency is 50%.
Table 2: Example of how the COP of a water-to-water heat pump varies with the distribution/return temperature.
|Heat distribution system (supply/return temperature)||COP|
|Conventional radiators (60/50 C)||2.5|
|Floor heating (35/30 C)||4.0|
|Modern radiators (45/35 C)||3.5|