The Principle
 

The Heat Pump Cycle

The Heat Pump transforms heat from a low temperature level into heat at a high temperature level at which it can be used for heating purposes. Even in wintertime with temperatures far below 0°C the Heat Pump can take energy from the environment.

This is performed by an endless "cooling" cycle. The cooling liquid evaporates at a very low temperature and takes a substantial amount of energy from the ambient (air, water, ground) when changing from a liquid to a gas phase.

The compresssor compresses this Refrigerant gas and thus brings it to a high temperature level. The hot gas is then condensated in a condenser where it transforms into a liquid state and gives the heat to the heating system. Then the refrigerant / working fluid is expanded again when passing through an expansion valve so that the circular process can continue.  The Heat Pump extracts stored solar heat from the environment - ground, water or air - and delivers it plus the driving power in the form of heat to the heating and hot water circuits.

Fig.: The cooling circle of the Heat Pump with typical pressure and temperature values. Refrigerant R134a, A7/W50.

Kältekreislauf der Wärmepumpe

 

Coefficient of Performance (COP)

COP = amount of heat delivered / drive power = (envirnmental energy + drive power) / drive power

The coefficient of performance COP (E) indicates the delivered amount of heat relative to the drive power required.

A coefficient of performance of 4 therefore means that quadruple the used electrical energy is disposable as usable thermal output. The coefficient of performance is a instantaneous value.

The yearly performance figure results from the supplied energy in relation to the electrical driving power required for the entire heating season. It is the average intergrated value of all COP values accumulated over a period of a year.

 

Carnot Cycle

The Heat Pump cycle follows more or less the (ideal) Carnot cycle of a combustion engine in the reversed direction . Thus we can also calculate the COP by taking the temperature difference between heat source (evaporator) and heat sink (condenser):

ec = T / (T - Tu) = T / DT

ec = coefficient of performance according the ideal Carnot cycle
Tu = temperature of the environment from which the heat is taken from(cold side) up to
T = temperature of the heat sink to which the heat is transferred (warm side)
DT = temperature difference between warm and cold side

A representation of the values of the variables T (temperature) and S (entropy), going through during the carnot cycle, looks as follows:

Fig.: T-S Diagramm.
The curve consists of two adiabatic curves (S = const) and two isotherms (T = const)

Der Carnot-Prozeß im T-S Diagramm Energy taken from the environment: Surface a

Driving power compressor: Surface b

Total delivered energy: Surface a + b

S = entropy = energy content

4 - 1: evaporation 1 - 2: temperature rise(stroke) during compression
2 - 3: condensation 3 - 4: expansion

Example:
Tu = 0°C = 273 K, T = 50°C = 323 K
ec = T / (T - Tu) = 323 / 323-273 = 6,46

Ideal processes are not possible. However the Coefficient Of Performance of the Heat Pump process   including the losses, will be actually lower. Due to thermal, mechanical and electrical losses, as well as the power requirement of the auxiliary pump (eg brine circulating) the effectively achieved COP or E is smaller than Ec.

For rough estimate E can be set equal to 0,5 x Ec
 

The temperature lift determines the COP

In all cases the Coefficient Of Performance depends on the temperature difference between the heat source and the heat use: The lower the required temperature lift is, the more efficient and economical the Heat Pump works. Therefore the optimal design of the entire installation is very important.

 

Working Liquid / Refrigerant

Substances are suitable as working liquid or refrigerant if they have a high specific energy content and   need a high level of energy for evaporation. In our new millenium only substances free from chlorine are permitted. These do not have any ozone-depletion-potential (ODP = 0). R 134 a, R 407 C, and propane fulfill these conditions.

OCHSNER uses the unflammable safety refrigerants R 134 a and R 407 C only. The Esther oil used is thereby biologically degradable. This means the Heat Pump can be installed at any location and without any restriction. In contrary Heat Pumps with inflammable media however have restrictions and numerous safety guidelines to follow.


© OCHSNER Wärmepumpen GmbH