Q&A, Aquathermal and Thermal Energy from Drinking water (TED)

Last updated on: 5 January 2022

1. What is Aquathermal Energy?

Aquathermal energy is the collective name for thermal energy that can be obtained or recovered from water sources to provide heat and/or cooling capacity for commercial or domestic purposes. A distinction is made between thermal energy from surface water, from waste water and from drinking water. Each category can be further subdivided into different types of sources. In the case of thermal energy from drinking water, for example, a distinction can be made between untreated water and clean water[1].

Aquathermal energy is a ‘very low temperature heat source’ (10-30°C)[2]. This means that a heat pump is needed to bring this heat to a usable temperature for space heating purposes[3]. The required temperature depends on the level of insulation (energy label) of the building to be heated.

By applying aquathermal energy in combination with an open subsurface energy system (OBES[4]), heat extracted in the summer can be used in the winter as a replacement for fossil energy sources. 

2. How can a drinking water company supply heating or cooling capacity?

To fully understand the potential of TED, it is first important to understand the temperature of the drinking water in the mains system. The temperature of drinking water is not constant as it passes through the mains. Studies have shown that drinking water takes on the soil temperature by the time it reaches the customer[5]. This is due to heat exchange with the soil when the water is in the mains. The soil affects the drinking water temperature least in large pipelines and most in the smaller pipes in the mains. Depending on the source (groundwater, surface water) and the season, drinking water therefore heats up or cools down on its way from the source to the customer.

From the perspective of supplying heat, this means that the mains system acts as a ‘collector of soil energy’ in the summer.

A water company can only make heating or cooling capacity available as long as that does not interfere with its main job of supplying drinking water. Heating or cooling can be provided in practice by directing part of the volume flow in a pipe through a heat exchanger[6] and, after heat exchange, mixing it back into the rest of the water in the pipe[7]. When heat is extracted from drinking water, the temperature of the drinking water falls. When drinking water is used for cooling, the temperature of the water increases.

3. Is TED safe for water quality?

Yes: research has shown that the current application of TED has no negative consequences for drinking water quality[8]. On the contrary, when heat is removed, water quality may benefit because microbiological processes in the drinking water are slowed down temporarily as a result. There are areas requiring consideration. They are covered under question 7.

Effects of TED should always be considered in the local context. In one situation, cooling with drinking water resulting in the heating up of the water, may be undesirable (for example in the summer in the mains system) while, in another situation, it is not a problem.

In the case of all the TED systems that have been studied in the past in BTO and TKI projects (see footnote no. 7), the outgoing temperature (after mixing in the drinking water pipe) did not exceed 15°C[9]. In the WarmingUP research programme[10], KWR is conducting research into the possible impact of TED at higher temperatures[11]. The results of this research are expected in 2022.

4. Can TED also be used to supply cooling capacity?

Yes: depending on the situation, TED can be a source of sustainable cold. Examples include the existing TED system at Sanquin in Amsterdam (Waternet)[12] and the TED system for cooling in the shopping centre The Mall of the Netherlands (Dunea)[13]. In both systems, cold is ‘extracted’ during the winter season and stored in an ATES system for cooling in the summer.

5.Is the potential of TED really relevant for the heat transition?

Yes, even though the potential supply of heating or cooling capacity from TED varies considerably depending on the location. The potential is determined primarily by the volume flow at a given location and the temperature differential[14] that can be applied. A map showing the potential of TED by local area is available on the publicly available  Aquathermal Viewer.

A good guiding principle is that larger transport pipelines are always useful for the application of TED if there is a client in the vicinity. However, distribution pipes can also be useful if there is enough[15] drinking water flowing through them. Because drinking water in distribution pipes generally heats up more than in transport pipelines in the summer, a larger temperature differential is usually available in distribution pipes than in transport pipelines.

6. Can TED be used as a solution for the undesirable heating up of drinking water in summer?

No. Locally, heat extraction using TED in summer can have a positive effect on drinking water quality[16]. However, this is a local and temporary effect because the warmer soil causes the water to heat up again before it reaches the customer. TED is therefore not a systematic ‘source-to-tap’ solution for the unwanted heating up of drinking water as a result of climate change or sources that give off a lot of heat to the ground[17] such as high- or medium-temperature heat networks.

7. What are the technical boundary conditions for TED?

The following areas require attention in the development and application of TED:

• To safeguard drinking water quality, a double-walled plate heat exchanger in combination with a higher pressure on the drinking water side with respect to the secondary circuit (the side of heat extraction) is required. Furthermore, the changed drinking water temperature should not have a negative impact on drinking water quality[18]. In other words, when TED is used, the requirements for drinking water quality from the Drinking Water Decree must still be met.
• Application of materials and chemicals All materials used in the TED system that could come into contact with drinking water must therefore receive quality certification recognised by the statutory authority[19].
  All detergents and disinfectants used to clean and disinfect the primary part of the TED system must have a certificate[20].
• Monitoring A TED system should be monitored at various locations upstream and downstream of the heat exchanger in terms of temperature, pressure and volume flow measurements.
• There must be adequate expertise at the water company about the operation and control of TED systems, even when a third party is managing the heat supply.

The ‘Aquathermal Energy Configurations’ report was drafted, with a chapter on TED, as part of the WarmingUP research programme, This report can be downloaded here.

8. How has TED been used until now?

As this Q&A goes to press, there are twelve operational TED systems in the Netherlands. In addition, some systems are in the preparation or implementation phases. The systems have a number of similar features but there are also differences:

Similarities:

• All the current TED systems use plate heat exchangers for the exchange of thermal energy with the drinking water and they are therefore basically similar in terms of the technical design.
• Temperature change In all current systems, the net temperature change in the downstream drinking water line[21] after passing through the TED system is limited to a maximum of approximately 2°C.

Differences:

• Heating or cooling Most systems deliver either cooling or heating capacity only. There are also some systems that can supply both.
• Most systems deliver heating or cooling capacity to non-residential buildings such as offices or commercial premises. Some systems supply domestic heating or cooling capacity.
• Capacity There are considerable variations in the size of existing TED systems. Thermal capacity varies between about 45 kW for the smallest system to about 2,500 kW for the largest.
• Seasonal storage Some systems use seasonal storage to extract heat in summer or cold in winter and draw on it in the winter or summer respectively.

The actual configuration of a TED system is always customised and it depends on the local conditions for both the demand (customer) and supply of heating or cooling capacity (drinking water).

9. Does heat extraction with TED noticeably lead to extra costs for hot tap water for the drinking water customer?

No: research[22] has shown that, for a net[23] temperature reduction of 1-2°C in the mains, any additional heating costs for downstream customers are negligible (a few euros a year at most). For the majority of downstream customers, there is no measurable temperature change at all because the soil around the drinking water pipes neutralises the temperature change before the drinking water reaches the customer[24]. This effect depends on how long it takes the water to travel from the TED system to the client. Any measurable effects of TED must be considered in relation to the natural variation in the temperature when the drinking water reaches the customer. This variation results from the changing seasons: in late summer, the water is warmer than in spring. Any residual temperature change due to TED when the water reaches the customer is small, and negligible with respect to this natural variation in drinking water temperature throughout the year.

10. Does heat extraction with TED result in additional carbon emissions elsewhere?

Possibly, although this depends very much on where the TED system is located. At the level of the drinking water customer, as in the case of possible additional costs (question 9), the additional carbon emissions per customer are marginal. At the system level, the ultimate carbon saving of a TED system far outweighs any additional total carbon emissions downstream of that system[22].

To ensure that the carbon savings outweigh any additional carbon emissions, it is advisable to assess the preliminary design of a TED system test on the basis of this criterion.

11. Can TED be used without using seasonal heat storage?

Yes: TED is possible without the application of open subsurface energy (OBES)[25]. However, in most cases, this will affect the potential supply of heating or cooling capacity. If seasonal storage is not used, the potential of TED will generally be lower. It is simply the case that more cooling capacity can be supplied in the winter and more heating capacity in the summer, with the reverse applying to demand for heating and cooling. Using TED without seasonal storage does have benefits for the business case: no wells need to be bored for the OBES system. Furthermore, boring is not permitted everywhere: in areas where boring is not permitted or in groundwater extraction areas, for example, OBES construction is not allowed.

From the perspective of seasonal heat storage (OBES), aquathermal energy is an interesting and sustainable source for ‘regeneration’. OBES systems have to be balanced. This means that as much heat must be extracted from the soil (over a given period of time) as is added. In the case of most buildings, demand for cooling in the summer does not correspond precisely to demand for heat in the winter. An OBES system therefore needs a ‘regeneration device’ to maintain the thermal balance. The advantage of TED over thermal energy from surface water is that the infrastructure (the drinking water mains) is already in place. As a result, in the case of TED in built-up areas, the costly construction of heat infrastructure is required to only a limited extent; all that is needed is a ‘bypass’ of a few dozen metres in the mains system that is already present below the road anyway.

The combination ‘TED/OBES’ is therefore interesting from both the perspective of TED (potential) and from the perspective of OBES (sustainable source for regeneration ‘next door’). Dunea Warmte en Koude, Waternet and KWR have conducted cost-benefit analyses in recent years from the perspective of the entire life cycle (Total Cost of Ownership; TCO). These show that regeneration with TED is cheaper in the long run than, for example, the use of dry coolers.

When TED is used without OBES, a daily buffer is still required. This is because the volume flow in a drinking water line fluctuates considerably during the course of the day due to the fluctuation in drinking water demand (specific transport pipelines excepted). As a result, the supply of heating or cooling capacity from TED is very dynamic.

Footnotes

• [1] For a general introduction to drinking water infrastructure and TED, see the report ‘Berekening potentieel TED; uitgangspunten en achtergronden’
• [2] Terms such as ‘high’, ‘medium’, and ‘low’ temperature are often debated. Here, the temperature range has been taken from the brochure ‘Warmtenetten Ontrafeld’ from TKI Urban Energy;
• [3] Aquathermal energy is therefore fully sustainable only when the heat pump is powered by green electricity.
• [4] Open Subsurface Energy System, also known as ‘ATES’.
• [5] Blokker, E.J.M. et al. (2017), Drinkwatertemperatuur, bedreigingen en kansen, TVVL Magazine 46 (2017), pagina 16-18, KWR, Nieuwegein.
• [6] Also known as a ‘counter-current device’.
• [7] Technically, other options can be imagined for heat exchange in which the drinking water itself does not come into contact with the heat exchanger, a ‘winding tube’ being one example. However, these methods are not currently used in practice and there are drawbacks in terms of (1) access to the water line in the event of a burst pipe (conflict between primary and secondary task) and (2) the efficiency of heat exchange; winding tubes are much less efficient in this respect than plate heat exchangers. Furthermore, it is debatable in the case of winding tubes to what extent aquathermal energy is actually involved.
• [8] Bloemendal, Van Bel, N. et al. (2018), Verdieping Warmte en Koude uit Drinkwater, Rapport nr. BTO 2017.072, KWR, Nieuwegein.
• Moerman, A. et al. (2019), Aquathermie in Tilburg: warmte en koude uit drinkwater; Evaluatie TED-installatie bij Fontys Hogeschool, Rapport nr. KWR 2019.021, KWR, Nieuwegein.
• Ahmad, J.I. et al. (2020), Effects of cold recovery technology on the microbial drinking water quality in unchlorinated distribution systems Environmental Research, Volume 183, 109175.
• [9] This was the temperature in the bypass connecting the heat exchanger of the TED system to the mains. The temperature of the drinking water exiting the heat exchanger and after mixing with the main flow was therefore lower.
• [10] www.warmingup.info
• [11] https://www.kwrwater.nl/en/projecten/thermal-energy-from-drinking-water-ted/
• [12] https://www.waternet.nl/nieuws/2017/juli/waternet-levert-kou-aan-bloedbank/
• [13] https://www.dunea.nl/algemeen/projecten-en-innovatie/mall-of-the-netherlands
• [14] The actual applicable temperature differential at a given location is determined by the drinking water temperature at the location and the characteristics of the potential customer.
• [15] As a rule of thumb (at the time of drafting this Q&A), an average volume flow of approximately 10 m³/h can be adopted for this purpose (given average daily consumption).
• [16] Bloemendal, Van Bel, N. et al. (2018), Verdieping Warmte en Koude uit Drinkwater, Rapport nr. BTO 2017.072, KWR, Nieuwegein.
  It should be pointed out here that a TED system does not make the water in a consumer’s tap cooler because the soil heats the drinking water up again before it reaches the customer.
• Moerman, A. et al. (2019), Aquathermie in Tilburg: warmte en koude uit drinkwater; Evaluatie TED-installatie bij Fontys Hogeschool, Rapport nr. KWR 2019.021, KWR, Nieuwegein.
• [17] https://www.kwrwater.nl/en/projecten/climate-proof-drinking-water-now-and-in-the-future/
• [18] This point requires some nuance in practice for two reasons: (1) there is not yet any scientific evidence for the effects of TED in the temperature range 15 – 25°C. Research is ongoing in this area – see footnotes 6 and 11 – and (2) there is already natural variation in the temperature of drinking water when it reaches the customer. The discussion about the induced temperature change should always be seen in this context.
• [19] Meerkerk, J. (2017), PCD 3 Richtlijn drinkwaterleidingen buiten gebouwen; Ontwerp, aanleg en beheer (gebaseerd op NEN-EN 805:2000) (oktober 2020), Rapport nr. PCD 3:2020, KWR, Nieuwegein.
• [20] For a current overview of certificated chemicals and agents: https://www.praktijkcodesdrinkwater.nl/certificatie/gecertificeerde-producten-en-processen/overzicht/
• [21] In other words: after the mixture of the separate flow past the heat exchanger and the main flow in the transport pipeline or distribution pipe.
  The temperature change in the separate flow may therefore be higher but it may never exceed 25°C.
• [22] Moerman, A. et al. (2019), Aquathermie in Tilburg: warmte en koude uit drinkwater; Evaluatie TED-installatie bij Fontys Hogeschool, Rapport nr. KWR 2019.021, KWR, Nieuwegein.
  Blokker, E.J.M. et al. (2013), Thermal energy from drinking water and cost benefit analysis for an entire city. Journal of Water and Climate Change, 4(1): p. 11-16.
• [23] In other words: after mixing the separate flow from the heat exchanger with the main flow.
• [24] Vice-versa, therefore, there is also no cost benefit for the customer when drinking water is heated in the case of cold extraction.
• [25] Open Subsurface Energy System, also known as ‘ATES’.