Reversible heat pump
- Ruben Gielen
- Bram Van Bogaert
- Kristof Janssens
Working principle
Reversible heat pumps work similarly to regular heat pump, but they can reverse the heat pump cycle such that they can provide both heating and cooling to the internal space. Using a reversing valve, the refrigerant’s flow direction is reversed, the condenser coil becomes the evaporator coil and vice versa.
When working in heating mode, the outdoor coil acts as the evaporator and heats the refrigerant at low temperature. The refrigerant passes through the reversing valve towards the compressor. The indoor coil acts as the condenser and cools down the refrigerant, transferring its heat toward the secondary circuit, where it will be used to heat the system.
When working in cooling mode, the reversing valve changes its position, reversing the refrigerant’s flow cycle. The outdoor coil now acts as the condenser that dissipates heat, cooling down the refrigerant coming from the compressor. The indoor coil becomes the evaporator, where the refrigerant will absorb heat, cooling the system.
The reversible heat pump thus can both heat and cool, but it can only provide one at a time. Only the indoor coil is connected to the circuit, which implies that the heating flow and the cooling flow use the same pipe system. This makes the behavior of the reversible heat pump very different from the behavior of the heat pump for heating and cooling (see Heat pump for heating and cooling ), where the heat pump is connected to the heating circuit on the evaporator side and connected to the cooling circuit on the condenser side.
Reversible heat pump Base Circuit
The reversible heat pump BC represents the principle discussed above. Note that the indoor coil has two connections: The heating circuit and the cooling circuit. Two pairs of control valves ensure that the cooling circuit is closed off when the reversible heat pump works in heating mode, and that the heating circuit is closed off when the reversible heat pump works in cooling mode.
The reversible heat pump BC itself has four parameters:
Environment temperature: The temperature of the room/environment in which the heat exchanger is placed. This parameter is used to take heat losses from the heat pump to its environment into account during simulation. This is an optional parameter. If no value is filled in, the outside temperature is taken by default.
Design configuration: This is an important parameter, as it defines the method the Hysopt software will use to size the heat pump. There are three options:
Heating and cooling demand (Highest) (Formerly known as electric demand): In this mode, the Optimiser will determine the size of the reversible heat pump based on the minimal electric power that is needed to satisfy both heating and cooling conditions. Based on the design heating and cooling flows, the required compressor power for each situation is calculated using the COP & Power tables. The situation that requires the highest compressor power is used to size the reversible heat pump.
Heating demand: In this mode, the Optimiser will determine the size of the reversible heat pump to meet the thermal installed capacity requirement for the heating condition.
Cooling demand: In this mode, the Optimiser will determine the size of the reversible heat pump to meet the thermal installed capacity requirement for the cooling condition.
The Heating and cooling demand (Highest) mode cannot be used when locking the parameters ‘Design power’, ‘Reference evaporator temperature’, or ‘reference condenser temperature’.
See below for an elaborated example of how the different design configurations affect the sizing of the heat pump.
Simulation configuration: This parameter defines whether the heating or cooling mode will be prioritized. There are thus two options:
Heat leading: In this mode, the Optimiser will prioritize heating over cooling. When the reversible heat pump is fed a heating and a cooling signal that are both larger than zero, the Optimiser will activate the heating mode and de-activate the cooling mode.
Cold leading: In this mode, the Optimiser will prioritize cooling over heating. When the reversible heat pump is fed a heating and a cooling signal that are both larger than zero, the Optimiser will activate the cooling mode and de-activate the heating mode.
See below for an elaborated example of how the different simulation configurations affect the behavior of the heat pump.
UA value supply pipes: This parameter determines the heat losses on the very short pipe section that is integrated within the reversible heat pump BC at the supply side of the two way on/off valves. By default, this parameter is set to 0 W/K.
The reversible heat pump BC additionally has three more components:
Heat pump: Representing the parameters of the actual heat pump itself. The heat pump component has nine parameters. For an in-depth description of those parameters, see Heat pump.
KV value indoor coil: A parameter for the pressure drop at the indoor coil (the condenser in heating mode) of the reversible heat pump. The KV value expresses the amount of flow that is needed for a pressure drop of 1 bar. This must be manually filled in by the user, as its value will affect the pressure drop propagated upstream during the ‘optimise components’ step. By default, this value is set at 100.
UA value indoor coil: A parameter for the heat losses at the indoor coil (the condenser in heating mode) of the reversible heat pump to the environment. These heat losses depend on the temperature difference of the heat pump’s indoor coil temperature and the environment temperature. By default, this parameter is set to 0 W/K.
Capacitance indoor coil: A parameter for the heat inertia of the indoor coil (the condenser in heating mode) of the reversible heat pump. The capacitance represents the amount of energy that is needed to increase the coil’s temperature by one Kelvin. By default, this parameter is set to 10 000 J/K.
Compressor configuration: A parameter that determines how the reversible heat pump can modulate its electrical input. The heat pump’s compressor can either be controlled by a variable speed drive, or by compressor stages. By default, this parameter is set to a VSD drive with a minimal modulation of 0 % and a start-up delay of 0 s.
COP table: The table that the Hysopt software uses to determine the heat flow of the reversible heat pump. based on the electrical input power, the reference temperatures and the modulation signal.
Power table: The table that the Hysopt software uses to determine the electrical input power of the reversible heat pump, based on the reference temperatures and the modulation signal.
Design power: The thermal design power of the reversible heat pump. To define whether this parameter belongs to heating or cooling, the right mode has to be selected at the design configuration parameter.
Reference evaporator temperature: The reference air temperature at the outdoor coil for which the reversible heat pump’s design power is defined in heating mode, or the reference supply temperature at the indoor coil for which the reversible heat pump’s design power is defined in cooling mode.
Reference condenser temperature: The reference supply temperature at the indoor coil for which the reversible heat pump’s design power is defined in heating mode, or the reference air temperature at the outdoor coil for which the reversible heat pump’s design power is defined in cooling mode.
Two way on-off valve for heating: The control valve that opens & closes the heating circuit. For more information on its parameters, see Control valves.
Two way on-off valve for cooling: The control valve that opens & closes the cooling circuit. For more information on its parameters, see Control valves.
Impact of the design configuration on the calculated heat pump size
For the same hydronic system, the choice of the design configuration can highly impact the results of your model. In this section, we’ll investigate the differences between the three different modes. We do this investigation two times: once without locking the design parameters of the heat pump, and once after locking the design parameters of the heat pump.
We’ll analyze the following model:
The heating circuit has a design flow of 50 kW and a temperature regime of 80/60 °C. The cooling circuit has a design flow of 100 kW and a temperature regime of 10/16 °C.