Heat pump guide – general aspects

HEAT PUMPS – INTRODUCTION

Limited fuel resources and the worldwide orientation towards environmental protection are aspects that have drawn attention to the possibility of using renewable energy sources. Today, these heat pumps represent safe, efficient and innovative heating equipment, with economical operation in terms of electricity consumption.

Heat pumps – are equipment that provide the necessary technical prerequisites to efficiently use the solar energy accumulated in underground water, in the soil or in the air, in the form of ecological heat, for heating or cooling premises and for preparing domestic hot water.

The heat pump obtains approximately three quarters of the required energy from the surrounding environment, and for the rest, the heat pump uses electric current as drive 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 these equipments.

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)
• by means of a compressor (electrically driven) the heat pump increases the temperature of the heat agent taken from the environment to the temperature required in the home heating system
• with a heat pump, the solar energy accumulated in the environment can be used all year round!

1.2 Independence from fossil fuels

The energy sources used by the heat pump are available right on our doorstep, completely 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, wall heating, low-temperature heating systems) guarantees a comfortable and healthy climate.

Reversible heat pump models (water-water or soil-water) can also provide cooling needs in summer.

Heat pump systems are generally very quiet, fully automated and do not require periodic maintenance operations.

There is no need for a fuel depot, 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 increasingly also systems using 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 heat pump 3/4 of the energy consumed is and remains free.

4. Safe operation

Heat pumps produce thermal energy through a thermodynamic cycle, without burning fuel. This aspect considerably reduces the risk of accidents! Moreover, heat pumps work with non-flammable refrigerants.

5. Ideal both for new buildings and for the rehabilitation of existing buildings

Heat pumps can be used for heating and cooling new buildings and constructions with low energy consumption (where most conventional systems are not available or are not convenient to implement technically or economically due to low thermal powers). Also, where there is already a modern heating system that uses fossil fuels and a cost reduction is desired, heat pumps can be used as additional heating systems (bivalent operation).

6. Multiple functions

Heat pumps can provide heating throughout the cold season, cooling throughout the warm season (with minor modifications) and domestic hot water throughout the year.

7. Ecological

The burning of fossil fuels for heating homes and offices represents today one of the biggest sources of CO2 production. Heat pumps produce thermal energy without pollutants using energy from the environment.

HOW DOES A HEAT PUMP WORK?

The operation of a heat pump – a simple principle with exceptional results!

Regardless of their type, these heat pumps can be seen as equipment that increases the temperature of a working environment using an additional amount of energy to produce useful energy.

The way a heat pump works is basically the same as that of a piece of equipment we use every day: the refrigerator. The same technique, only with reversed use; in the case of the refrigerator, the cooling agent takes the heat from the food and gives it to the environment. The heat pump takes the 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)

In the evaporator there is a liquid working agent at low pressure (refrigerant). This is a substance that has a low boiling point. The temperature of the source (soil, water or air) is higher than the boiling temperature corresponding to the pressure of the refrigerant. This temperature difference leads to the transmission of heat from the environment to the working agent, and it boils and vaporizes. The heat required 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 vaporizer by the compressor. The refrigerant is compressed until it reaches the temperature required for heating and preparation of domestic hot water.

The compression process is essential for the efficiency of a heat pump. For the entire range of heat pumps, Compliant Scroll compressors are used, they consist of two spirals (one fixed and one movable) that continuously compress the working agent. Compliant compressors are completely hermetic, 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 installation

The vapors of the working agent (refrigerant) reach the condenser of the heat pump, which is surrounded by the heat agent. The temperature of the thermal agent is lower than the condensing temperature of the working agent, so the vapors cool and condense.

The energy (heat) taken by the vaporizer plus the heat generated during the compression process (in the compressor) is released in the condenser and transferred to the heat agent in the form of useful energy for heating.

4. Expansion valve – the circuit closes

The working agent is later returned to the vaporizer, through an expansion valve. Thus, the working agent passes from the high pressure of the condenser to the low pressure of the evaporator. At the entrance to the vaporizer, the initial pressure and temperature values ​​are reached. The circuit is thus closed.

HEAT PUMPS – MAIN COMPONENTS
WHERE DO WE GET HEAT?

Soil, water and air are elements available in unlimited quantities to be used as a source for a heat pump.

In each individual case, the most advantageous energy source depends on local circumstances, the location of the building and its heat requirements.

For their practical use, energy sources must meet several conditions:

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

The ground

The soil has the property that it can accumulate and maintain solar energy over 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 performance coefficient.

The temperature in the soil is between 7 and 13°C throughout the year (at a depth of 2 m).
The heat taken from the ambient environment is transmitted to the vaporizer of the soil-water heat pump through a water-antifreeze mixture (salt water); the freezing point of this solution is approximately -15°C.
The heat accumulated in the soil is taken through horizontally mounted heat exchangers - also called soil collectors - or through vertically mounted heat exchangers - soil probes.

Collectors placed in the ground - horizontal collectors

The heat from the soil is taken by means of plastic tubes - polyethylene - mounted in the soil on a large surface.

The tubes are placed parallel, in the ground, at a depth of 1,2 to 1,5m and depending on the diameter of the tube, at a distance of approx. 0,3 to 0,7m, so that on each square meter of capture surface approx. 1,43 to 2m of tube.

The amount of heat that can be used and therefore the size of the required surface 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 amounts of mineral components and the size of the pores filled with air. 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 smaller the number of pores. The values ​​of the specific extraction power for soil fall 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 carried out starting with the second half of the heating season through solar radiation and more abundant precipitation, so it is necessary to be able to ensure that the soil "accumulator" is ready for heating again for the next season.

Soil probes

Due to the large areas of land required for the installation of horizontal collectors, sometimes it is difficult to realize the system for reasons of space.

For small land areas, soil probes are an alternative to the collector placed horizontally in the soil. They can be inserted at depths of 50 to 150m.

Probes are usually made of polyethylene tubes and four parallel tubes are usually mounted (double tube probe with U profile).

The water-antifreeze mixture flows to the lowest level through two pipes and returns to the heat pump evaporator through the other two. In this way, the frame is taken from the ground, along the entire length of the tubes. The spaces between the tubes and the soil must be filled with a material with good thermal conductivity (bentonite).

The extraction power differs greatly, between 20 and 100 W/m probe length.

Ground water

Groundwater is also a good accumulator for solar energy. Even in the coldest winter days, it has a temperature between 7 and 12°C. However, ground water is not available in sufficient quantities and at a suitable quality in all areas.

To use the heat, two wells must be made: one suction and one absorbing (draining); a distance of at least 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 absorbing one.

Water from lakes and rivers is also suitable for use as a heat source, because they also act as a heat accumulator.

The air

Air is the cheapest option when used as a source for a heat pump.

Air-to-water heat pumps use the outside air as a heat source, which is directed through air ducts, by a fan built into the device, to the evaporator, which extracts the heat from the air.

Waste heat

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

The name of a heat pump is given by the working medium 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.

*Soil-water heat pumps can also be found under the name "salt water-water heat pumps". This name comes from the medium used on the primary circuit (source) for heat transfer; for this, a mixture of water and antifreeze (tyfocor) is used, called "brine" in English or "sole" in German. The operating mode of the heat pumps adapts to the existing heating system in the building, in the case of older buildings, for which modernizations are made. In this case, account must be taken of the maximum temperature that the heat pumps can achieve per cycle (between 55 and 65°C).

For systems already sized above this temperature level, the heat pumps can only work together with another heat generator. In new buildings, you can choose the heat distribution system. In this case, taking into account the highest annual outdoor temperature parameters, a heating system with a maximum temperature per round of 35°C will be chosen (floor heating, walls, etc.).

From a technical point of view, the following operating regimes can be distinguished:

• The monovalent operating mode – the heat pump must provide the entire building's heating needs as the only heat generator
• Mono-energy mode of operation – the heat pump is used in combination with another heating system that works with 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 heat pump system, the most important factors are the performance coefficient and the annual performance factor.

Performance coefficient and annual performance factor

The ratio between the usable thermal energy and the electrical drive energy taken over by the compressor is called "momentary power index" or "performance coefficient".

Coefficient 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.

Empirical formula:

• One degree increase in the temperature in the heating circuit leads to a decrease in the COP by 2,5%
• The increase of one degree in the source temperature leads to the increase of the COP by 2,7%.