Chiller as a heat pump

Chiller as a heat pump

Heat energy can be taken from groundwater at a temperature of +10 °C and the heat output (Qo + W) obtained can be used for operation heating. The cooling function in the groundwater is unimportant. In addition to the heat, which is available in the order of magnitude of the cooling capacity, the drive energy from the motor drive is also used because the electrical energy used also converts to heat.

The advantage of using a chiller as a heat pump is that no heat through conversion of energy - such. B. by combustion processes o. Ä. - Is generated, but that already existing in the surrounding heat by raising the temperature potential is used.

This is an exceptionally energy-efficient, cost-effective and environmentally friendly way of generating heat.

For the energy efficiency of the process, the temperature of the waste heat to be recovered is important because the coefficient of performance ε of the refrigerator is influenced directly by the condensation temperature. B. a floor heating with a heating medium temperature of +30 ° C are charged, so rich +35 ° C condensation temperature. If a process z. B. heated with +50 ° C, the condensation temperature must be +55 ° C. This makes the energy efficiency already significantly lower. (The following calculation examples will show this.)

Before that, however, we introduce the heat output number εCW (theoretical Carnot process related) in order to better assess the efficiency of this process. The cooling capacity number εC refers to the cooling capacity and is calculated from the quotient of cooling capacity by drive energy.

Here we add the drive energy to the cooling capacity and then divide by the drive power. So we get the statement how much electrical energy has to be applied to get the desired heating energy. In other words, how much heating energy can be generated from one kilowatt hour of electrical energy? To get a realistic view, not only the drive power of the compressor must be included in the calculation, but also the electric power of all auxiliary equipment such as pumps and fans. In addition, the different operating conditions over the year must be observed. For reasons of clarity, this is not done here.

A compressor is connected to the cooling of the groundwater by means of an evaporator on the cold side. The evaporation temperature is +2 ° C. As a floor heating system is charged, the condensation temperature is +35 ° C.

The chosen compressor provides a cooling capacity of 85.9 kW with an electrical power consumption of 20.9 kW. 
The refrigeration capacity (εC) is thus 4.11.

The heat output coefficient results from (cooling capacity + drive) / drive, thus:

This means that with the use of 1 kWh of electrical energy you can achieve 5.11 kWh of heating energy!

A heat pump thus makes it possible to remove heating energy (in this case 4.11 kWh or 80% of the total heating power) almost free of charge from the underground! If the heat pump is now powered by renewable energy, the result is a completely emission-free heating method.

If a condensation of +55 ° C is required, the cooling capacity is reduced to 2.6 with a cooling capacity of 66 kW and a drive power of 25.2 kW. The heat output number is then still 3.6.

The amortization of the method is calculated very simply from the fact that z. B. fuel oil during combustion provides a heat output of 10.2 kWh / L. Taking into account losses in the boilers, combustion can not be more favorable than 9 kWh / L.

In this example, therefore, 106 kWh of heat * with only 20.9 kWh of electricity can be made available. Divided by the calorific value of the heating oil, this results in a heating oil requirement of 106/9 = 11.8 liters per hour for the same heat output.

With an assumed fuel oil price of 0.90 € per liter, the price is: 0.90 x 11.8 liters = 10.62 €. On the other hand, if the electric current costs € 0.20 per kWh, and if the same heating power is used, then 20.9 kWh of electricity will be used, then the operating price will be € 4.18.

The savings are accordingly: 
(€ 10.62 to € 4.18) = € 6.44 per operating hour.

These are economically convincing reasons why chillers are increasingly used as heat pumps where a good heat source is available, with z. B. the removal of heat - in addition to the removal from the groundwater - can also be done from wastewater streams.

Of course, the energy saving depends very much on the development of the price of oil or gas and electricity. For such calculations, one therefore depends on predictions (estimates) of the price development, and the calculation results are subject to a certain degree of uncertainty.

Examples from practice (just to name a few) are:

• Heating of swimming pools from groundwater
• Building heating by means of warm air in factories
• Production of process heat for industrial purposes

Source:

Reisner, Klaus; Reisner, Timo

Expertise Refrigeration
An introduction to education and training with tasks and solutions

6th, revised and expanded edition 2016 
ISBN 978-3-8007-4098-7, e-book: ISBN 978-3-8007-4099-4

VDE VERLAG GmbH, Berlin - Offenbach