Heat Pump is the device that "pumps" (transfers) heat, usually from a low temperature heat source to a receiver of higher temperature than the source. The heat pump is designed to transfer heat (thermal energy) contrary to the natural flow, which is from the hotter to the cooler. Its operation requires the consumption of mechanical work which requires electricity consumption. The operation principle of the heat pump is applied to refrigerators, chillers, air conditioners and recently to water heating.

Heat pumps consist of a pump, a compressor, an expansion valve and 2 heat exchangers. The most common heat sources for such machines are the ambient air and the ground. Depending on the source’s and the receiver’s nature, respectively, heat pumps are divided into air-air, air-water, ground-air, ground-water and water-water.

The ratio of heat transferred to the work consumed, is the specific degree of pump performance (COP, coefficient of performance) that depends on the mechanical characteristics of the heat pump and the properties of the refrigerant. In modern heat pumps we meet COP values ​​greater than 3.0, that classifies heat pumps as renewable energy technologies. COP value equal to 3.0 means that the pump consumes an amount of energy (usually electricity) to transfer a three times larger amount of thermal energy from the source to the receiver. So the particularity that makes the heat pump extremely effective is that it does not create heat like a boiler, various combustion devices or thermal accumulators but consumes a smaller amount of energy in order to carry otherwise unexploited heat from a colder source  into a warmer area (the receiver), when under normal circumstances it couldn't  be transfered.

A typical application of the heat pump is a combination with a geothermal heat exchanger in order to take advantage of the heat capacity of the ground. This will ensure that the medium wherefrom we will derive the heat in the winter will be in a constant temperature of 16^oC instead of 0^oC of the ambient air and that the medium to which we will reject the heat in summer will be in a constant temperature of 20^oC instead of 35^oC of the ambient air. And of course this medium is the ground. Thus, the reduction of temperature difference between the source and the receiver which leads to an increase in COP is the main reason why we use the ground heat exchanger.

VRV / VRF systems (Variable Refrigerant Volume / Variable Refrigerant Flow) operate like a heat pump with the difference that the mechanical work of the compressor is controlled by an inverter system. The temperature sensor which is embodied in the internal system unit detects and controls the room temperature by giving instructions to the inverter system. Inverter increases or decreases compressor's rotary speed by the selection of an appropriate current frequency. Actually, it is possible, to adjust the compressor's rpm in an electronic way, depending on the heating or cooling loads.

The inverter system selects the appropriate operation frequency of the air conditioner according to the room temperature, in other words it changes the performance of the air conditioner according to the loads. The unit operates at high frequencies when there is a big difference between the actual room temperature and the desired temperature and at low frequencies when the temperature difference is small. The Inverter selects the appropriate frequency based on the above temperature difference and executes the corresponding change in the compressor rotary speed.

VRV / VRF systems have significantly improved thermal performance in comparison with conventional heat pumps. Another important difference between the inverter air conditoner and a conventional air conditioner is the starting power. Furthermore, the low-temperature air is heated rapidly to the desired temperature. The time required for raising the temperature to the desired level is usually less than half of the time that is required for a conventional air conditioner. When the desired temperature is reached, the inverter gradually reduces its power. A low-power operation of the inverter air conditioner at 30Hz, maintains comfortable temperature, contrary to the conventional units that spend additional power with repeated ON-OFF operation at 50Hz.

Absorption chillers generate cooling from heat. The absorption technology can be applied whenever there is unexploited heat available and also a cooling load that needs to be met. Cogeneration systems can be combined with absorption chillers so that the heat generated can be used for the heating of buildings (e.g. hotels, hospitals, public buildings, etc.) during the winter and for the air-conditioning of buildings during the summer. Also, a very common heat source for absorption coolers is water that is heated by solar collectors during the summer (solar air conditioning). Finally, absorption technology can be applied with great success for industrial cooling using waste heat from adjacent industrial processes.

In general, the absorption cycle consists of four main stages:

1. The refrigerant that is used is water. By spraying water in a vessel, in vacuum conditions, the water evaporates causing a temperature drop. This temperature drop is exploited in order to chill water in the air-conditioner pipes.

2. The vapors that are produced are absorbed by an absorption medium. Absorbents that are used are lithium bromide (LiBr), ammonia or Silica Gel. Ammonia solution is used to achieve cooling temperatures below 0^oC.

3. The saturated solution of absorbent and water is regenerated by a heat source, which is water in temperatures above 70°C or low pressure steam.

4. The water vapors are released at a higher pressure and are cooled down by a coolant, usually water, in a temperature less than 35°C, in order to be condensed and begin a new cycle.