How does a water source heat pump work?
Heat pumps are an innovative way to make use of natural heat energy that is all around us in the ground, the atmosphere and even in water. They can use the heat already present to warm up your home without the use of gas, therefore reducing the amount of CO2 your home produces.
These amazing appliances can work even when the outside temperature is below zero. This is because, although it may feel cold outside to us, there is still some heat available that the pump can make use of. Additionally, the ground normally maintains a warmer temperature than the air, so even when outside temperature drops below zero, the ground could still maintain a mild 10 °C.
What is a water source heat pump?
A water source heat pump can use the natural heat that’s present in lakes, ponds, rivers, springs, wells or boreholes to heat your home. The heat that it sources can be heated further using electricity before being supplied to your taps, radiators and underfloor heating. A water source heat pump could replace your home’s gas boiler and it’s thought that heat pumps in general will increase in popularity over the coming years.
How a water source heat pump works
A water source heat pump works using a refrigerant to transfer heat from the water to your home. Before we go any further, there are two types of systems that you should know about: closed-loop and open-loop. A closed-loop system pipes water mixed with antifreeze through a series of coils or heat exchange panels that are submerged in the water source, such as a lake, although this will likely require planning consent due to the risk of water contamination. As the mixture travels through the coils, it absorbs energy from the water and delivers this straight to the heat pump. In open-loop systems, water is pumped directly from the lake and sent straight to the heat pump. Once the heat has been taken from the water, it can be pumped back into the lake.
Water is also a highly suitable heat source for heat pumps. Even on cold winter days, groundwater maintains a constant temperature of between 7 °C and 12 °C. In order to use groundwater for a heat pump, it must be extracted through a supply well and transported to the evaporator of a water/water heat pump. The cooled water is then routed through a return well.
Surface water (lakes or rivers), too, may be used as a heat source, although it should be noted that the temperatures will fluctuate quite dramatically depending on the season.
No matter what kind of system you have, heat will be pumped from the water to an intermediate plate heat exchanger. Via an intermediate circuit the energy is transferred to the heat pump’s heat exchanger, or evaporator, where it’s absorbed by a refrigerant. As the refrigerant heats up, it turns from a liquid into a gas. Refrigerant is a very powerful heat absorbent and can take in heat from very low temperatures. This is what makes heat pumps so suitable in cold climates.
Once the refrigerant has become a gas, it is compressed to increase the temperature to a more useful level and passed into a heat exchanger (condenser). In the condenser, the heat can be released to heat your water and radiators. When the gas begins to cool down, it condenses and turns back into a liquid. When this happens, it’s pumped back via an expansion valve to begin the whole process again.
One real benefit of water (and ground) source heat pumps in summer, is the option of passive cooling. It is also known as "natural cooling". This process transfers the cooler temperatures of the ground water directly into the house heating circuit.
For this, the heat pump compressor is idle. An additional heat exchanger provides the system separation. Natural cooling with water heat pumps is highly efficient, as it requires the operation of only two circulation pumps. During natural cooling, the heat pump will only be started to produce domestic hot water.
- Natural cooling is possible with the following systems:
- underfloor heating system,
- fan convectors,
- chilled ceilings,
- concrete core tempering.
With cooling via alternative heating surfaces the system is regulated so that condensation, i.e. undershooting the dew point on the heating surfaces, is avoided.
All Viessmann heat pumps are able to utilise natural cooling.
The closer the water source is to your home, the more beneficial the system will be for you. Otherwise, the pump could be using an excessive amount of electricity to pump the water from the source to your home. This means that the water source shouldn’t be located too high up either.
There are also certain environmental legislation that must be adhered to before the system is installed. An open-loop system could affect the temperature of the groundwater, so a license might be required from the Environment Agency.
How efficient is a water source heat pump?
The efficiency of a heat pump will depend on where it’s taking its heat from. An air source heat pump is the least efficient, followed by a ground source heat pump. A water source heat pump is usually the most efficient as the source temperature in winter is generally higher at a range of between 7 °C and 12 °C, however, lakes/rivers can also freeze in winter too depending on the geographic location and local weather patterns, therefore the source temperature could be less beneficial than it could be from the ground.
When moving water is used from either a stream or river the heat extracted is constantly replaced by warmer water that hasn’t been subject to local cooling from the heat pump operation. This can improve efficiency of the water source heat pump and can also prevent the water from freezing, an issue that can occur with still water.
A ground source heat pump will usually run at a Coefficient of Performance (CoP) of 4.8 (B/W of 0/35C) . This measurement calculates how efficient a heat pump is based on the amount of energy that’s inputted compared to the amount of energy that’s outputted. For example, if a water source heat pump uses one kW of electricity and three kW of heat is produced, the CoP will be four. A heat pump with a CoP of four will be more efficient than this. A water source heat pump generally has a CoP of around 6.0 (W/W 10/35C).
Can the water source freeze?
Heat pumps are sized to the heat demand of the home which is calculated by a professional installer/consultant. If the water source is too small, then the heat pump will reduce the temperature of the water too much so that it isn’t running as efficiently. In some cases, it could cause the water to freeze. When your heat pump is at specification stage, the installer should be able to determine whether the water source located near your house is suitable.