All about the heat pump

Termocasa

The heat pump dates from the beginning of the century. XX, with the invention of the refrigerator.

A series of phenomena and laws of physics compete at the basis of the operation of the heat pump: The XNUMXnd principle of thermodynamics: "Heat can never pass by itself from a body of lower temperature to one of higher temperature" (Clausius's statement), as well as the discovery of the physicist Watt: a compressed gas releases heat and vice versa, an expanded one - absorbs heat.

Why are we interested in the heat pump?

In the constant search for the best energy generation solutions, in this article we analyze the heat pump in all its variants.

All the efforts of specialists to find non-polluting or at least slightly polluting thermal resources and the transition to their mass use, for heating homes, drew attention to the implementation of heat transfer and the emergence of the increasingly famous heat pump. The heat pump is, due to its efficiency and the use of less polluting resources than other heating or cooling systems, one of the preferred solutions as an alternative installation system for projects, houses or buildings with almost zero consumption, and depending on the location situation is used for the active house.

What is a heat pump?

In a few words we can say that the heat pump attracts and transfers heat from one of the sources for which it is designed (whether we are talking about the ground, water or air) and with the help of a compressor, exactly on the principle of the refrigerator, raises the temperature of a refrigerant with a very low boiling point (somewhere around -2°C), transferring the temperature of the thermal agent that circulates through the heating installation regardless of the system to the static bodies used (heaters, floor, ventilation, etc.). The heat pump is characterized very simply by its operating mode very close to that of refrigerators with a compressor: a compressed gas releases heat and in the opposite direction, an expanded one absorbs heat.

How the heat pump works

The heat pump needs a permanent source of electricity. It is mandatory to connect to a permanent source, if possible to the public network, if not, preferably, due to the low consumption, to a solar photovoltaic panel system or, why not, to wind turbines, alternative systems, etc. The electrical source will power the compressor that compresses the coolant to reach the temperature used for heating and domestic hot water preparation. When the refrigerant vapor reaches the condenser in the heat pump, it will be surrounded by heat. The temperature of the heating agent is lower than the temperature of the cooling agent, in vapor state, so the vapor will cool and condense. At this moment, the heat is taken over by the vaporizer, including the heat generated during the compression process, a considerable amount of which is released in the condenser and transferred to the heat agent in the form of useful energy for heating. The transfer agent is then returned to the vaporizer using an expansion valve most of the time. The refrigerant thus passes from the high pressure of the condenser to the low pressure of the evaporator. With the entry into the vaporizer, here the values found at the beginning of the process are reached again, and the circuit closes.

Refrigerators, as well as air conditioners, are classic examples of heat pumps already existing in households and operating only in cooling mode. A refrigerator is really just a well-insulated box with a heat pump system connected to it. The vaporizer coil is located inside and is usually found in the compartment used for freezing. Heat is absorbed from the freezer and sent outside, usually behind or below the unit where the condenser coil is located. An air conditioner that transfers the heat from inside a house to the outside works in an almost similar way. So we have had the heat pump system in our homes for some time, but it has not been used both ways.

The heat pump can be used without the existence or support of other heat sources.

According to the criterion of operation, we meet heat pumps with regime

Monovalent: the heat pump is the only source of heating – it uses electricity from the public network or other sources to operate the compressor
Bivalent - parallel: it can be mounted and used in parallel with the heat pump and another heat source; when the parallel heat source is electricity, the system is called bivalent - monoenergetic parallel
Bivalent - alternative: the heat pump works alternatively with another heat source;
Bivalent – partial – parallel: the heat pump works in parallel or alternatively with another heat source.

The heat pump has a fully reversible cycle and can provide control over the indoor environment for heating in winter and cooling or dehumidification, if necessary, in summer. Since the ground or air outside always contains heat, why not use a heat pump to heat and cool the house? One of the principles used in the heat pump is that air at a temperature of -18°C contains approximately 85% of the energy it contains at 21°C.

What are the types of heat pumps?

There are two main types of heat pump, named according to the system used

Compression heat pump, which always works using mechanical energy through electrical energy
Absorption heat pump that can also run on heat as an energy source (via electricity or fuels).

A number of sources have been used as heat sources for heating.

Air source heat pump (extracts heat from outside air)

air-to-air heat pump (transfers the thermal energy of the indoor air)
air-to-water heat pump (transfers thermal energy to a water tank)

Geothermal heat pump (extracts heat from the ground or similar sources)

geothermal-air heat pump (transfers thermal energy to indoor air)
ground-to-air heat pump (the ground is the heat source)
rock-air heat pump (rock is heat source)
water-to-air heat pump (water as heat source)

Geothermal-water heat pump (transfers heat to a water tank)

ground-water heat pump (the ground is the heat source)
rock-water heat pump (rock is the heat source)
water-to-water heat pump (body of water as heat source)

Soil-Water heat pumps

The antifreeze solution called glycol circulates through the circuit of pipes buried in the ground of the heat source and takes the heat of the earth by means of a horizontal or vertical heat exchanger. This heat is transferred to the heating system of the house, with the help of a compressor and transported to the condenser. In the condenser, the water of the heat transfer system incorporates the heat absorbed in the evaporator and is supplied with energy to the compressor.

Depending on how the collectors are placed, we distinguish several types of heat pumps with collectors:

flat: they have the system of collectors in various shapes (U or spirals) buried at an average depth of 1-2m. It is an economic system of great efficiency, but it cannot be buried in any soil and it needs an extension almost double the habitable area.
of depth: they consist of probes mounted at a depth of 50-100m and have a very high efficiency. The installation costs are significant due to the drillings that have variable prices between 100-300 lei / meter. They are usable only if the soil allows it.
with direct vaporization: they have a special capture circuit in addition to the flat ones and for this reason they are more efficient or can occupy smaller spaces.

Water to water heat pumps

The heat from pumped groundwater or surface water is used in heat transfer, transported through the compressor to the condenser. Once there, the Soil-Water pump system is used. These are the most efficient using heat from geothermal sources with temperatures above 8 degrees Celsius, but also the most difficult to use both due to the availability of the resource and due to the legal regulations for its mining. The water-to-water heat pump has a particularly important role in industry or in the maximum exploitation of geothermal springs. Waste water or geothermal water with temp. maximums of 28-30ºC can be successfully exploited. In the case of geothermal waters, the source can be multiplied by using several PDCs in cascade. Obviously, the quality of the water will be taken into account, this impediment can be avoided by using suitable heat exchangers (anti-corrosive).

The water-to-water heat pump can also be used by exploiting water from lakes, rivers, tunnel waters, dams (which have temperatures >8ºC). The water-water system is also called an open-loop capture system. The speed of water flow through the vaporizer must not exceed 0,8m/s. This type of heat pump can achieve the highest COP of all the types we are referring to. Such a water-water system can easily reach a COP=5 and even exceed it if it is well made and correctly dimensioned. It can also provide impressive powers reaching thousands of kW, on a single unit or by connecting several smaller power units. However, until now, at least in Europe, it is not the most widespread heat pump. There are several reasons: the quality of the water must practically meet the quality of drinking water; the water extracted from the phreatic layers must be re-injected into the soil (the injection well must be located at least 15m downstream from the direction of water flow in the groundwater) for each thermal kW installed, a minimum water volume of 160 liters/hour is required , i.e. 0,16 mc/hour (at min. 8ºC), the flow having to be ensured at all times by the extraction well; in the EU there are very strict regulations regarding this type of drilling.

Air-Water heat pumps

Heat from outside air, supplied with a fan or air from the ventilation system, is transported through the evaporator to the heat transfer medium and then through the compressor to the condenser. This heat transfer system is made up of a liquid circuit, used for heating the home and/or for producing domestic hot water. These systems are not very efficient, in their current version the water-to-water heat pumps have the lowest efficiency ratio, so the cost is also lower. These pumps cannot work monovalently at temperatures below 15 degrees Celsius.

Air-Air heat pumps

Heat from outside air supplied with a fan or air from the ventilation system is delivered through the evaporator to the heat transfer medium and transported through the compressor to the condenser. The heat transfer system is composed of a system of air ducts or specific devices used to heat the building. They are used in rare cases, in heating systems, due to the low efficiency of the heat agent air (systems known generically as "air conditioning" are actually air-to-air heat pumps, and their COP is below 3).

Heat pumps with Ultraflex exchangers

The Ultraflex system no longer has an intermediate exchanger as found in most heat pumps on the market, which is why the system has a very high COP compared to the rest of the heat pumps, greatly increased reliability and very long operating and warranty terms extended, and their purchase price is very attractive. However, it has an extremely wide power variation (such systems are currently not limited in terms of power and offer unparalleled performance).

the size of the object to be heated (the habitable and heated surface, the average height of the rooms). The available (unbuilt) space around the objective;
the geographical area in which the objective is located;
the possibilities of installing other heating systems (gas, liquid fuel, liquefied gas, etc.);
the on-site possibilities of capitalizing on the "cold source" (water, air, soil);
the existence of other heating sources (to establish the operating mode of the heat pump);
electricity supply conditions (single- or three-phase network, maximum electrical power allowed on the electrical connection).

How much does the heat pump cost?

Without a doubt, the cost of using a heat pump heating system is significantly lower than that of a classic heating system. The difference comes from a consumption of 1kw of electrical power to the heat pump is transformed or can produce 3 – 5 kw in thermal power. A very good ratio compared to other classic solutions. The energy performance observed at the heat pump is given by the COP and is defined as the ratio between the amount of heat removed at the condenser and the electrical energy consumed by the compressor. From the point of view of environmental protection, compared to the heat pump but also considering the comfort of use, we can say that the residues left when heating with solid fuels are not at all negligible, nor the perspective of their constant and predicted price increase of natural gas, fuels liquids or electricity.

In various programs to encourage responsible consumption such as "Casa Verde", the owners who equipped their homes with a system based on the heat pump could benefit from funding from the state, the last time it was 8000 lei. The expenses related to the installation or commissioning of the system, as well as the value added tax, the VAT of these expenses, were also eligible for settlement.

The price of the Sol-Air type heat pump varies between 2400 euros for a power of approximately 7,5 Kw and can reach 12.000 euros for those of approximately 50 Kw. A price of approximately 280 euros / Kw can be considered depending on the need and the surface that is desired to be heated.

The price of the Water to water heat pump is average compared to the other heat pumps and is between 1900 euros for the small 6 Kw ones and can reach 15.000 euros for the 50 Kw ones. A price of approximately 260 euros / Kw can be considered depending on the need and the surface that is desired to be heated.

The price of the air-water heat pump is the lowest compared to the other heat pump systems and varies between 2100 euros / 6 Kw and can reach 9000 euros for 35 Kw. A price of approximately 240 euros / Kw can be considered depending on the need and the surface that is to be heated.

Other important observations

According to European standards, the calculation of the necessary thermal load is related to the square meter of surface, [W/m²], taking into account the maximum height of the room H=3m (typical height of most rooms – in atypical cases the necessary corrections will be made).

For example, if a building has a thermal requirement of 50W/m² and a habitable surface that needs to be heated of 350m² (H=3m), the thermal power of the thermal plant (regardless of its type) will be 250mpx50W/m² = 12500W=12.5kW . (The height is specified only in atypical cases).

Correct sizing of a pump is essential for its service life. An oversized heat pump, in addition to being more expensive, has an incorrect operating regime with more frequent starts and stops. An undersized heat pump, works longer and with small breaks. However, it is preferable, if there is no alternative, to undersize the pump (within certain limits) than to oversize it.

For a house of 200 square meters of usable area, where a ground-air heat pump of 8-10 Kw is sufficient, the total cost is around 60.000 lei and differs depending on the chosen brand.

Due to the relatively high price of heat pumps, it is uneconomical to heat poorly insulated spaces that require high thermal powers. It is preferable to insulate the building than to increase the power of the heating source. When determining the power of a reversible heat pump, the cooling capacity (EER) is taken into account. The cooling power is always greater than the heating power.

For example, the DIN 4701 norms regarding the conditions that heated buildings in Germany must meet are, in their evolution:

45-60 W/m2002 new constructions (XNUMX regulation)
50-60 W/m1995 new constructions (XNUMX regulation)
70-90 W/m1995 constructions made before XNUMX
120 W/mXNUMX old buildings made without any regulations

In the case of a building with very poor thermal insulation, the investment in a heat pump is not profitable.

Heat pump boilers

In addition to what was stated previously in this article, we believe it is important to treat heat pump boilers separately due to the wide coverage potential they offer. They work on the principle of air-air heat pumps, the heated water being stored in a boiler that forms a common body with the heat pump. The air-air heat pump has maximum performance when the ambient temperature is above 15 degrees Celsius, but it can also work at temperatures of -5 degrees Celsius. If the place where the boiler with the heat pump is installed is chosen correctly, in a room where there is also a thermal plant or other equipment that generates heat, from the refrigerator to servers, etc., the optimal operation of these boilers can be ensured with heat pumps, throughout the winter, with low costs. The boilers are also equipped with electric resistances for situations where rapid heating of water is required. Another useful option is that the exhaust air flow can be used to lower the air humidity or to cool the room in which the heat pump boiler is installed by a few degrees.

Sources used for this article

Remus Rădulet et al. Romanian Technical Lexicon, Bucharest: Technical Publishing House, 1957-1966.
The Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2004
Fundamentals of Engineering Thermodynamics, by Howell and Buckius, McGraw-Hill, New York
Heat pump – full details

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