Aquathermal energy refers to the process of extracting heat from surface water (the sea, a lake, pond, canal, stream, river, brook). The heat is extracted via a heat exchanger and an electrically driven heat pump is used to ramp the heat up to a temperature warm enough to provide heating and domestic hot water in buildings.

In many cases, the temperature of the surface water also allows you to cool the buildings in summer. If the electricity used is green, coming from PV-panels, wind turbines, small hydroelectric installations etc… the heating and cooling is also 100% renewable.

Scheme of an aquathermal energy installation. Cool production on the left, heat production on the right. Source: Community Energy: a practical guide to reclaiming power

How is heat extracted from surface water?

As mentioned, the sea, rivers, lakes, canals, … can be used as heat sources for heating buildings, districts, or even entire cities. This method of heating is renewable, dependable, and can greatly reduce the CO₂ emissions associated with heating buildings.

Aquathermal systems use existing, well established, heat pump technology to provide the desired heating to buildings and thus their reliability is comparable to more well know technologies such as geothermal or aerothermal heating.

The heat pump extracts the heat from the waterbody. This can be hard to comprehend because the temperature of a waterbody is much colder than the temperature of traditional heating being fed by a gas or oil boiler. Imagine a refrigerator in reverse, a heat pump extracts heat from the inside of your refrigerator and the heat leaves via the back. This is why the back of your fridge is so warm. Now in the case of aquathermal heating, instead of the inside of the refrigerator, the river is cooled. And instead of the heat leaving the back of the refrigerator, the heat is distributed throughout the building via the radiators or underfloor heating at a sufficiently high temperature.

An efficient technology

The efficiency of a heat pump is typically expressed using the term COP (coefficient of performance). A COP of 4 is similar to saying the heat pump is 400% efficient. This means that for every 1kWh of heat used to power the heat pump, 3 kWh of heat is generated for ‘free’

To put this into context an electric boiler or a plug-in electric radiator for example is 100% efficient, that means it has a COP of 1. So, for every €1 you spend on heating with direct electric heating you get €1 worth of heat. However, with an aquathermal energy heat pump system with a COP of 4 for every €1 you spend on heating you get €4 worth of heat.

In the right circumstances this can make for significant financial savings on operational expenses.

The efficiency of the water heat pump depends on a number of factors:

• The temperature difference between the temperature of the waterbody and the desired temperature for the interior heating system. E.g. it will be more efficient to bring water from for example 10°C tot 30°C than from 5°C to 60°C. The result will be that more electricity is needed when the difference between temperatures is higher.
• How well insulated the buildings are that the system is trying to heat/cool.
• How far the water needs to be pumped.

Within the extraction of heat from a waterbody, in general two main configurations can be used: open loop systems and closed loop systems.

Open systems

In an open loop system, there are four main steps to extract heat from a waterbody and upgrade it to the desired temperature:

1. In the first step water is pumped from the waterbody via a pre-filter to a technical room. Because waterbodies not only contain water, a pre-filter on the intake inside the waterbody is used to filter out particles (>1-4mm). The filter-screens of the pre-filter need to be self-cleaning with a backflush system (water is directed under high pressure against the filter screen so the particles that stick to the screen, detach themselves) to make sure manual maintenance is limited.
2. In a second step water is filtered to an even finer level (<0,05-1mm). The fine filtering step is essential to increase the lifetime of the heat exchanger. Different fine-filtering components/methods can be used, e.g. disc-filters, scrape filters, …
3. In the third step the heat is extracted from the water. This is done by a heat exchanger, located in the technical room. After the heat exchange takes place in the heat exchanger, the temperature of the water is still at the source temperature (e.g. 5°C in wintertime).
4. In the fourth step the temperature of the water is increased by a heat pump using (renewable) electricity as described above.

Once the heat is extracted from the source water, it is discharged back into the waterbody. National or regional regulation defines how much heat can be extracted from the waterbody. Typically, a 3°C or 5°C temperature difference is allowed. The actual effect on the temperature of the waterbody will however be smaller since right after the discharge mixing and regeneration occurs with the much higher amount of water that remained in the water body.

Open systems are typically used in flowing waterbodies like rivers since the obstruction of the waterbody caused by the system is very limited. A minimal obstruction in flowing waterbodies is mostly desired since boats are otherwise hindered, or flowing particles can get stuck.

Closed system

The main difference between a closed system and an open system, is that for a closed system the heat exchanger is placed directly inside the waterbody. A closed system just consists of a heat exchanger in the water and a heat pump on land connected in a closed loop of piping. Therefore, no filtering is needed which makes the operation efficiency higher and cheaper. In the closed system a heat exchange fluid (a mixture of antifreeze and water) circulates between the heat exchanger and the heat pump.

The heat exchanger located in the waterbody has a larger volume than a plate heat exchanger in an open system. Since the closed system therefore takes up more space in the waterbody, these systems are typically used where obstructions are not a critical aspect, like lakes, ponds, or inside existing hydropower installations on rivers.

Where can aquathermal systems be used?

Aquathermal systems can be used in small scale applications, like for example buildings located next to a lake, a castle surrounded by a moat, etc.

In larger scale projects, like district heating networks, aquathermal systems are normally used in conjunction with a mix of different technologies. As with most renewable energy technologies, there will be some cases where one technology alone cannot provide what is needed so a mix may be required. This ensures a consistent supply that is financially viable. Since many cities are located on the coast, along rivers, lakes, etc., aquathermal energy can play a very important role in the transition to a sustainable heating.

Existing aquathermal project sizes range from a few kW’s up to several MW’s.