HEAT PUMPS - INTRODUCTION

Limited fuel resources and global focus on environmental protection are issues that have drawn attention to the possibility of using renewable energy sources. Today, these heat pumps represents safe, efficient and innovative heating equipment, with economic operation from the point of view of electricity consumption.

Heat pumps - are equipments that offer the necessary technical premises to efficiently use the solar energy accumulated in groundwater, soil or air, in the form of ecological heat, for heating or cooling the premises and for the preparation of domestic hot water.

Heat pump obtains about three quarters of the required energy from the environment, and for the rest, heat pump uses electricity as the driving energy.
Modern heat pumps offer effective technical possibilities for reducing energy consumption and CO2 emissions. When modernizing old buildings, as well as new buildings, the heat pump is a good alternative.
This article deals with the basic principles of heat pump technology, the main technical variants and illustrates the most important aspects of the applications that integrate this equipment.

 WHY A HEAT PUMP?

 1. Economic motivation
1.1 Reduced operating costs
-Depending on the type of heat pump, up to 3/4 of the heating energy can be obtained from the environment (free of charge).
-Through a compressor (electrically operated!) Heat pump crHeat pumps - consumption evolutionis the temperature of the thermal agent taken from the environment to the temperature required in the heating system of the house.
-With a heat pump you can use all year round the solar energy accumulated in the environment!
1.2 Independence from fossil fuels
The energy sources used by the heat pump are available right at our door, totally independent of the availability or price of fossil fuels.

2. Comfort
The heating system with heat pumps offers the highest degree of comfort and the easiest operation. The heat distribution system normally used in combination with heat pumps (underfloor heating, walls, low temperature heating systems) guarantees a comfortable and healthy climate.
Reversible heat pump models (water-water or ground-water) can also provide the necessary cooling during the summer.
Heat pump systems are generally very quiet, fully automated and do not require periodic maintenance operations.
There is no need for a fuel tank, no ash removal and no chimney cleaning.
3. Security for the future
Choosing a heating system is a decision for many years. Heat pumps are the most modern heating technology available today.
   Today, heat pumps are not only replacing heating systems with wood, liquid fuel or coal, but more and more often systems that use natural gas.
In addition, there is the question: will we be able to afford the costs of the heating system in 20 years?
With each increase in fossil fuel prices, the cost of heating with heat pumps becomes more advantageous compared to heating with gas, liquid fuels or pellets.
Regardless of the increase in the price of electricity with a 3/4 heat pump, the energy consumed is and remains free.
4. Safe operation
Heat pumps produce thermal energy through a thermodynamic cycle, without burning fuel.
This considerably reduces the risk of accidents! Moreover, heat pumps work with non-flammable refrigerants.
5. Ideal for both new buildings and for the rehabilitation of existing buildings
Heat pumps can be used to heat and cool new buildings and low-energy buildings (where most conventional systems are not available or are not convenient to implement technically or economically due to low thermal power).
Also, where there is already a modern heating system that uses fossil fuels and a reduction in costs is desired, heat pumps can be used as additional heating systems (bivalent operation).
Heat pumps - energy without pollutants
6. Multiple functions
Heat pumps can provide heating throughout the cold season, cooling throughout the hot season (with minor modifications) and domestic hot water throughout the year.
7. Ecological
The burning of fossil fuels for heating homes and offices is today one of the largest sources of CO2 production. Heat pumps produce pollutant-free thermal energy using energy from the environment.

 HOW DOES A HEAT PUMP WORK?

Operation of a heat pumps - a simple principle, with exceptional results
   Regardless of their type, these heat pumps can be seen as equipment that raises the temperature of a working environment using a certain amount of energy.Heat pumps - operation additional, to produce useful energy.
   The operation of a heat pump is basically the same as that of an equipment that we use every day: the refrigerator.
   The same technique, only with reversed use; in the case of the refrigerator, the coolant takes the heat from the food and gives it to the environment. The heat pump takes heat from the environment (soil, water or air) and transfers it to the heating system in the form of thermal energy.

 1. The vaporizer - taking heat from the environment (soil, water, air)Heat pumps - evaporator
   In the evaporator there is a liquid working agent at low pressure (refrigerant). This is a substance that has a low boiling point. The source temperature (soil, water or air) is higher than the boiling temperature corresponding to the refrigerant pressure. This temperature difference leads to the transmission of heat from the environment to the working agent, and it boils and vaporizes. The heat needed to vaporize it comes from the external heat source (soil, water, air).

2. The compressor - temperature rise
   The vapors resulting from the working agent are continuously sucked from the evaporator by the compressor. The refrigerant is compressed until it reaches the temperature necessary for heating and preparation of domestic hot water.
   The compression process is essential for the efficiency of a heat pump.
   Compliant Scroll compressors are used for the whole range of heat pumps; they consist of two spirals (one fixed and one movable) that continuously compress the working agent.
Compliant compressors are completely airtight, have a much longer life and are quieter than the piston model used in the past for heat pumps.

3. The capacitor
- Heat transfer to the heating system The vapors of the working agent (refrigerant) reach the condenser of the heat pump, which is surrounded by heat. The temperature of the heating medium is lower than the condensing temperature of the working agent, so that the vapors cool and condense.
   The energy (heat) taken by the evaporator plus the heat generated during the compression process (in the compressor) is released in the condenser and transferred to the thermal agent in the form of energy useful for heating.

4. Expansion valve - the circuit closes
   The working agent is then returned to the evaporator through an expansion valve. Thus, the working agent changes from the high pressure of the condenser to the low pressure of the evaporator. At the entrance to the evaporator, the initial values ​​of pressure and temperature are reached. The circuit is thus closed.

 HEAT PUMPS - MAIN COMPONENT
 WHERE DO WE GET THE HEAT?

Soil, water and air are elements available in unlimited quantities to be used as a source for a heat pump.
In each case, the most advantageous source of energy depends onHeat pumps - heat sources the local circumstances, the location of the building and its heat demand.
For their practical use, energy sources must meet several conditions:

  • availability in sufficient quantity
  • storage capacity as high as possible
  • temperature level as high as possible
  • sufficient regeneration
  • economic uptake

The ground

Heat pumps - temperature variationThe soil has the property that it can accumulate and maintain solar energy for a longer period of time, which leads to an approximately constant temperature level throughout the year and thus to the operation of heat pumps with a high coefficient of performance.
The soil temperature is between 7 and 13 ° C for a whole year (at a depth of 2 m).
The heat taken from the environment is transmitted to the evaporator of the ground-water heat pump through a mixture of water-frost protection agent (salt water); the freezing point of this solution is about -15 ° C.
The heat accumulated in the soil is taken up by horizontally mounted heat exchangers - also called ground collectors - or by vertically mounted heat exchangers - ground probes.

Collectors placed in the ground - horizontal collectors
   The heat is taken from the ground by means of plastic - polyethylene tubes mounted in the ground on a large surface.Heat pumps - collectors
   The tubes are placed parallel, in the ground, at a depth of 1.2 to 1.5 m and depending on the diameter of the tube, at a distance of approx. 0,3 to 0,7 m, so that on each square meter of the catchment area to be mounted approx. 1,43 to 2 m of tube.
   The amount of heat that can be used and therefore the size of the surface required depends very much on the quality of the soil. Regarding this aspect, the determining quantities are: first of all the amount of water in the soil, the quantities of mineral components and the size of the pores filled with air. The accumulation capacity and thermal conductivity are higher the more the soil is moistened with water and the higher the amount of mineral components and the lower the number of pores.
   The values ​​of the specific extraction power for the soil are between 10 and 35 W / m2.
   When using horizontal collectors, plants with very deep roots should not be planted around the tubes. Soil regeneration is already done starting with the second half of the heating season by solar radiation and more abundant rainfall, so it is necessary to ensure that for the next season the "battery" soil is ready again for heating.

Soil probes
 Heat pumps - soil probes  Due to the large areas of land required for mounting horizontal collectors, it is sometimes difficult to build the system for reasons of space.
   For small land areas, soil probes are an alternative to the collector placed horizontally in the ground. They can be introduced at depths of 50 to 150 m.
   The probes are usually made of polyethylene tubes and usually four parallel tubes are mounted (double tube probe with U profile).
   The water-frost protection agent mixture flows to the lowest level through two tubes and returns to the heat pump evaporator through the other two. This takes the heat from the ground, along the entire length of the tubes. The spaces between the tubes and the ground must be filled with a material with a good thermal conductivity (bentonite).
   The extraction power differs a lot, between 20 and 100 W / m probe length.

Groundwater
   Groundwater is also a good battery for solar energy. Even in the coldest days of winter it has a temperature between 7 and 12 ° C.
   However, groundwater is not available in sufficient quantities and at an appropriate quality in all areas.
   To use the heat, two wells must be made: one suction and one absorbent (draining); a minimum distance of 5 meters must be provided between them, and the location must be chosen so that the direction of water flow is from the suction well to the absorbent one.
   Water from lakes and rivers is also indicated for use as a heat source, because they also act as a heat accumulator.

The air
   Air is the cheapest option to use as a source for a heat pump.
Air heat pumps - water uses as heat source the outside air, which is directed through air ducts, by a fan built into the appliance, to the evaporator, which extracts heat from the air.    

Residual heat
   Of the sources that can be used with a heat pump, residual heat is the most efficient, ensuring the highest performance parameters. However, it has the disadvantage of a very limited availability.

Heat pumps - namesThe name of a heat pump is given by the working environment on the primary and secondary circuit.
By primary circuit we mean here the heat source (air, soil, water), and the secondary circuit is the heating installation.
* Ground-to-water heat pumps can also be found under the name "salt-water heat pumps". This name comes from the environment used on the primary circuit (source) for heat transfer; for this a mixture of water and antifreeze agent (tyfocor) is used, called "brine" in English or "sole" in German. The operating regime of the heat pumps is adapted to the existing heating system in the building, in the case of buildings old, for which modernizations are made.
In this case, the maximum temperature that the heat pumps can achieve during the flow (between 55 and 65 ° C) must be taken into account.
For systems already dimensioned above this temperature level, the heat pumps can only work together with another heat generator. For new buildings you can choose the heat distribution system. In this case, a heating system with a maximum flow temperature of 35 ° C (underfloor heating, walls, etc.) will be chosen, taking into account the highest annual outdoor temperature parameters.
Heat pumps - heat source
From a technical point of view, the following operating regimes can be differentiated:

  • Monovalent operating mode - the heat pump must ensure as the only heat generator the entire heating requirement of the building.
  • Monoenergetic operating mode - heat pump is used in combination with another heating system that runs on electricity.
  • Bivalent operating mode - the heat pump is used in combination with another heat source that works with solid, liquid or gaseous fuel.

For the evaluation of a heat pump or a complete system with heat pump, the most important factors are the coefficient of performance and the annual performance factor.

Performance coefficient and annual performance factor
The ratio between the usable thermal energy and the operating electric energy taken over by the compressor is called "current power index" or "performance coefficient".
Heat pump - coefficient of performanceCoefficient of performance (COP) = specified by the manufacturer, laboratory value
Annual Performance Factor (FPA) = the ratio between the heat extracted during a year and the total energy consumed in a year
In general, the coefficient of performance increases as the temperature difference between the source and the heating system decreases.
Heat pumps - graph coefficient of performanceEmpirical formula:

  • Increasing the temperature in the heating circuit by one degree leads to a decrease in COP by 2.5%
  • A one degree increase in the source temperature leads to an increase in COP by 2.7%.
HEAT PUMPS - NAMES
HEAT PUMPS - FUNCTIONING MODES
HEAT PUMPS - PERFORMANCE FACTORS