A heat pump is a device Thermodynamique making it possible to transfer heat from the coldest medium (and thus to still cool it) about the hottest middle (and thus to heat it), whereas spontaneously heat is diffused hotter towards coldest until the equality of the temperatures.

The Réfrigérateur is the most known system of heat pump. The air-conditioner is another current system of heat pump. Both are used to produce “cold” more than of heat: heat that they generate is lost. The Sueur is a natural refreshing CAP.

But the term of “heat pump” (CAP) was especially diffused to indicate the heat pump Géothermique or the heat pump air-water, system of domestic heating popularized in France following the First oil crisis in 1973. In certain countries whose France, tax incentives support the diffusion of certain models of heat pump air-water. These systems regain in popularity since the year 2000, in particular because of construction many swimming pool S.

Principle

The Principle of operation of an CAP is identical to that of a refrigerator.

The CAP recovers (if heating) or evacuates (if air-conditioning) the energy delivered by the change of state (liquid/gas) of the fluid used (cf enthalpy); the source far away from the habitat evacuates or recovers the calories of the opposite change of state (gas/liquid). The " engine compresseur" ensure the circulation of the fluid in the " échangeurs" holders; who maintain the changes state insofar as the calories are recovered or evacuated (exchange unit external habitat/unit). In the contrary case, the changes of state could not occur and the output of the CAP would be only of 1, that is to say the heating output of the compressor alone (what arrives for example if it is too cold or too heat).

One defines the effectiveness $\backslash \; eta$ of an CAP by the report/ratio of the “useful” energy, the heat restored (for the heating), on expensive energy, the work provided to the CAP.

$$

\ eta = - \ frac {\ hot Q_ {\ mathrm {}}} {\ W} \ qquad \ textrm {with} \ qquad \ Q < 0 \ quad \ textrm {and} \ quad \ W > 0 \ quad \ textrm {and} \ quad 0 \; \ Leq \; \ eta \; \ Leq \; \ infty

The effectiveness of a heat pump decrease with the variation in temperature, and is limited by the Second law of thermodynamics.

In practice, the salesmen of heat pumps in general announce the relationship between the thermal power of their machine and his electricity consumption. One gives him by convention the name of coefficient of performance or COP . Moreover, of the technical constraints the lowest temperature and the difference in temperature limit: impossible to reject water with less 0°C for example, limitations of flow by the pump and the pipes, or of effectiveness of the exchangers for the difference in temperature.

$$

COP_ {\ mathrm {heat}} = \ frac {\ hot Q_ {\ mathrm {}}} {\ Delta has} \ Leq \ frac {hot T_ {\ mathrm {}}} {hot T_ {\ mathrm {}} - cold T_ {\ mathrm {}}} = \ frac {1} {\ eta_ {\ mathrm {cycle Carnot}}}

$$

COP_ {\ mathrm {cooling}} = \ frac {\ cold Q_ {\ mathrm {}}} {\ Delta has} \ Leq \ frac {cold T_ {\ mathrm {}}} {hot T_ {\ mathrm {}} - cold T_ {\ mathrm {}}}

The Cycle of Carnot is the cycle ditherme having the best effectiveness. The temperatures T are expressed in Kelvin. T°K = T°C + 273, is for example T = 290°K for 17°C.

Calculation of the COP heat: for a domestic heating, the theoretical maximum is about 15 (in practice, the COP of the machines currently on sale is of 3 or 4).

Various systems

Certain “CAP” work on the heat of the air (air-to-air or air-water), others with the heat of water (water-water, more rarely water-air), or of the ground. The CAP using the heat of the ground are called geothermic heat pump . This name can lend to confusion with the geothermic district heating which uses heat at high temperature of the deep basement, but it is a very different system.

The heating on the ground in the dwelling, alternate with the traditional radiators, can be supplied with a traditional circuit of heating at low temperature, by a heat pump on the air, water and/or by a thermodynamic system extracting heat from a sensor of ground, often of the buried pipes on average 70 centimetres under the ground, in general in the garden. These systems are used to transfer from the energy of the ground towards a dwelling, for the requirements in heating the winter. These systems are economic compared to the price of installation of an CAP but less powerful.

There are also other heat pumps which use the air as source (cooling of the air to heat water of swimming pool for example) but the output is less good and the period of operation is more restricted, the risks of icing being important when the temperature of the surrounding air is low and the high hygroscopy. The investment is on the other hand much less important.

The COP is about 5 on the thermodynamic models and 3 for the systems air-water installed at present (that wants to say that for 1 kw of electricity consumed, the house will receive 5 kw or 3 kw of heat). The COP of any heat pump increases with the temperature of the cold source and decreases with that of the hot source, it can reach 5 to 7 in summer for water of swimming pool (air with 25°C for water with 28°C) but lower than 3 in winter (the standardized values given by the manufacturers are for an air with 7°C and water of heating with 35°C). The COP has significance only at one temperature of cold source and of hot source given, it can never be equal to 1 same for the very low outside temperatures (<-15°). The curve of COP according to the outside temperature and water of heating is thus to take into account to decide installation appropriateness and to evaluate the performances.

The systems sold with the general public have a thermal power from 15 to 20 kw what is equivalent to the low range of powers of the boilers to the gas or with the Fioul. For reasons of comfort a chimney with insert is used very occasionally.

Certain models are reversible, i.e. able to transfer from heat of the house towards the cold source. These machines have the advantage of being able to be used as Climatisation if the receivers of heat lend themselves to it: the heating floor has a capacity limited to become refreshing floor, the radiators are not appropriate, it should be replaced by ventilo-convectors definitely more expensive.

The air-to-air heat pumps can theoretically use the air resulting from a Canadian Puits (or Puits of Provence) to feed the air intake and to thus improve their output. In practice quantity of reduced air circulating very strongly this interest: the well Canadian or of Provence is effective only with one flow and a limited speed of air.

Operation of the CAP

The geothermic heat pump, so called domestic Geothermics , uses the heat contained in the ground to feed a network of heating like a floor heating or the radiators. See below Catchment circuit.

This principle known since a score of years, underwent notable technical evolutions which enable him to compete with the means of heating " traditionnels". An CAP known as reversible makes it possible in particular the floor to become refreshing in summer period.

The heat pump with water uses a source of water: well, river, lake, brook, subterranean water, it should be checked that this source is available in sufficient quantity, that its use is authorized (administration of water and sanitary services) and that the cooled water rejection or return is carried out under conditions acceptable for the environment.

The heat pump with air uses the ambient air, always available in abundance and without problems of rejection, but its mixing can be noisy and its temperature is very variable, and precisely unfavourable when the needs for heating are largest.

Catchment circuit

The apparatus of the CAP

The apparatus, which takes heat with the cold source thanks to the catchment circuit, has 4 principal bodies (cf diagram opposite):

• 1) the condenser : the refrigerant releases its heat in the dwelling while passing from the gas state in the liquid state.
• 2) the pressure reducer : it prepares the reaction of vaporization by lowering the pressure of the liquid.
• 3) the evaporator : the heat taken with the cold source vaporizes the refrigerant.
• 4) the compressing : actuated by an electrical motor, it raises the pressure of the refrigerant by compressing it.

There exist two different technologies:

• the direct relaxation : it is composed of only one circuit. The refrigerant passes directly in the ground heating or the convectors. The catchment circuit plays the part of evaporator and that of heating ensures condensation.
• fluid intermediaries : it has a circuit separated for collecting, the heat pump and the heating. This system is a little more expensive, but more powerful, in particular for cooling, and it uses less refrigerant.

There exist also analog and digital systems.

The circuit of heating

One uses mainly three types of transmitters of heating:

• the Floor radiating low temperature

• the ventilo-convectors
• radiators low temperature

It is important to retain that the heat pumps offer an output interesting only to the condition of being connected to transmitters dimentionnées for low temperatures. Indeed, the coefficient of performance announced by certain publicities at water temperature high are purely whimsical. The radiators can sometimes be re-used if they are adapted to the heating at low temperature; it can be the case for old installations dimensioned for an operation in Thermosiphon: dimensions of drains and radiators can make it possible to heat at low temperature with a flow much higher than that of the thermosiphon thanks to the modern accelerators. It is also possible of redimentionner certain radiators according to the need specific to the room concerned in order to compensate for the fall of temperature by a surface of higher emission. It is the solution adopted most usually in the event of restoration of an existing installation.

Cycle thermodynamic

The fluid circulating in a heat pump undergoes a cycle of transformation made up of 4 stages:

• At the exit of the compressing , the fluid is in gas form with high pressure and its temperature is high.
• In the condenser, the fluid passes in the liquid state and yields energy which is transferred towards outside (circuit of heating) in the form of heat.
• At the exit of the Condenser, the fluid strongly sees its temperature decreasing.
• In the pressure reducer, the energy of the fluid (its enthalpy) remains constant.
• At the exit of the Pressure reducer, the fluid is in the liquid state low pressure and low temperature.
• In the evaporator, the fluid recovers energy in the form of heat (catchment circuit) to avoid congelation. The pressure remains constant and the fluid becomes completely gas.
• At the exit of the evaporator, the fluid is moderate with approximately 5°C and low pressure.
• In the compressor, the gas is compressed and thus passes from a low pressure to a higher pressure thanks to the mechanical energy provided by the compressor. This transformation can be done at constant temperature.

Market

A total of: 53510 domestic heat pumps were installed in France in 2006, against only a one thousand in 1997. This figure makes it possible this country to become the second European market for this apparatus behind the Sweden but in front of the Germany and the Suisse. However, in the Scandinavian country, 95% of the new houses are equipped with it, against only 10% in France, where however the market doubles one year value on the other.

Notes and references of the article

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