By Biatur Mandia, MEng
This module explores some of the key factors that successfully decrease carbon emissions in domestic hot water systems (DHW). The CPD is focused on a low temperature system powered with hydrogen blends. A practical example is utilised to further explain the benefits of such systems.
Domestic hot water (DHW) plays a significant role in the total energy used in buildings. While the energy used for space heating or lighting has decreased over the past years, the energy used for DHW has increased considerably in the last 20 years. This can be attributed to new patterns in hygiene and comfort. For instance, the yearly DHW consumption per capita in Denmark has increased from 10 m³/year to 15 m³/year over the last 20 years¹. Therefore, it is important to evaluate changes to how the hot water is supplied to improve the efficiency of the system.
Low temperature domestic hot water system
Low temperature DHW systems operate in the range of 40° to 55°C. This presents several key benefits that help to reduce the carbon emissions of the system, and therefore, contribute to achieving the net-zero target by 2050.
On the other hand, low working temperatures present important challenges due to hygiene and reduced comfort. The hygiene requirements are associated with the control of legionella, a potentially fatal form of pneumonia. Specific guidance for the control of hot water systems is provided in the Health and Safety Executive Approved Code of Practice (ACOPL8) and its associated regulations, HSG274 Part 2. ACOP L8. Some key guidelines are summarised in table 2 (below).
The legionella safety requirements differ in situations. These largely depend on the volume of water required during peak times. Generally, household applications have low water volumes and can be more flexible in terms of temperature range and safety requirements. These applications often do not need recirculation and storage when instantaneous heaters are employed.
Commercial applications have high volumes of hot water. These require high working temperatures to satisfy the
The consumption of a commercial DHW system can be categorised as follows:
A lot of research is investigating the feasibility of having low-temperature return systems. There is a growing interest that instantaneous water heater could soon reduce the outlet temperature to 50°. Instantaneous heaters are considered “low risk” for legionella by the H&S executive and new guidelines are due to be published on the CIBSE Knowledge Portal³.
A successful low-temperature DHW system was implemented at Great Ormond Street Hospital. This was achieved by having a rigid disinfection process in place. Copper-silver ionisation measures were used to clean the water of the system. The working temperature was set to 43° and no legionella issues were found4.
Practical example: recirculation of a low temperature DHW system powered by hydrogen
In the proposed example, hot water is utilised on-demand, and thus, eliminating the need of storing high-temperature water, as for ACOPL8. The system is adjusted to include copper-silver ionisation which further reduces the risk of legionella. The working temperature is set to 43°. In the proposed system, the legionella risk was minimised by eliminating any major water storage. The appliance can supply temperatures in the range of 42-45° without affecting the efficiency of the water heater. This approach can be considered as the first step towards Lean Energy and the elimination of hot water storage often referred to as waste or “Muda” in Lean Japanese principles5.
The working principle of the system is shown by the schematic diagram in Figure 1. The water heater is powered by the hydrogen and natural gas blend. This reduces the carbon emission of the appliance. The heater also modulates the heating input according to the desired outlet temperature. For instance, if the heater has a gross heating output of 58.3kW and a 13:1 turn down ratio, this can potentially modulate down the heating to 4.4kW, thus enabling massive savings in the secondary return system (recirculation) while still providing the desired temperature. The heating process is optimised and programmed using a high-tech processor and PCB; performance charts are programmed to operate the appliance at maximum efficiency. Advanced control strategies can enable significant energy efficiency improvement of the DHW production systems and generate carbon savings6.
Heat loss savings of low temperature system vs high temperature analysis
The energy losses in supplying domestic hot water can be categorised into two main groups:
Low temperature DHW can reduce the amount of heat loss of the system. The following analysis shows the heat loss
savings of a pipe distribution network.
Consider the formula from the heat loss
Equation 1 – Q = 2 π K L ( t1 – t2 )
ln (r2 / r1)
It is assumed that geometric and material properties remain constant. The same system is used at two different temperatures, high (55) and low (43). All the other parameters in equation 1, except for t1, are constant, and can therefore be represented by a constant, as shown in equation 2.
Equation 2 – 2 π K L = a
ln ( r2 / r1 )
Substitute Equation 2 into Equation 1 – Q = 2 π K L ( t1 – t2 ) = a (t1 – t2 )
ln( r2 / r1 )
Substitute the temperature values of the two systems, high temperature 55° and low temperature 43°. The ambient temperature is 15.
Q1 = a ( 55 – 15 ) = 40a Q2 = a ( 43 – 15 ) = 28a
The heat loss savings in a pipe when operating a low temperature compared to a high temperate can be found as follows:
Q1 – Q2 (100) = 40a – 28a (100) = 30% savings in heat loss – indicative value
Carbon savings of the proposed system
The following example considers a business case for a small/medium size application. The analysis shows the savings in carbon when a low temperature system powered by hydrogen blends is employed. The savings in heat loss previously calculated were also added in the analysis. The hydrogen blend was assumed to start in 2025. The results have shown carbon savings in the range of 33% and 50% compared to a high temperature system. The exact value would depend on the mass flow rate of the proposed system and other parameters.
Low-temperature systems are effective ways to reduce the carbon footprint and hydrogen is currently undergoing
unprecedented political and business momentum. Gateshead has become the first UK community to receive hydrogen blends via the public natural gas network.
This growing trend is very likely to continue over the next years. The carbon savings of the proposed system are considerate and as a comparison, these equate to the annual average carbon emissions of 20 cars. With thousands of commercial buildings in the market, such implementation would have a very positive impact on the environment and our future.
1 Pomianowski, M. Z., Johra, H., Marszal-Pomianowska, A., & Zhang, C. (2020). Sustainable and energy efficient domestic hot water systems: A review. Renewable and Sustainable Energy Reviews, 128, 109900.
2 Yang, X., Li, H., & Svendsen, S. (2016). Energy, economy, and exergy evaluations of the solutions for supplying domestic hot water from low temperature district heating in Denmark. Energy conversion and management, 122, 142-152.
3 July 2021, Turning on energy savings for hot water: instantaneous hot water, CIBSE Journal https://www.cibsejournal.com/uncategorized/turning-onenergy-savings-for-hot-waterinstantaneous-hot-water
4 Cloutman-Green, Elaine, et al. “Controlling Legionella pneumonia in water systems at reduced hot water temperatures with copper and silver ionization.” American Journal of Infection Control 47.7 (2019): 761-766.
5 Demyanova, O. V., Badrieva, R. R., & Andreychenko, I. S. Features of lean manufacturing in the energy sector.
6 Dentz, J., Ansanelli, E., Henderson, H., & Varshney, K. (2016). Control Strategies to Reduce the Energy Consumption of Central Domestic Hot Water Systems (No. NREL/SR-5500-64541; DOE/GO-102016-4703).
National Renewable Energy Lab. (NREL), Golden, CO (United States).
7 Lutz, J. (2005). Estimating Energy and Water Losses in Residential Hot Water Distribution Systems (No. LBNL-57199). Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US).
8 The Engineering toolbox – Uninsulated Cylinder or Pipe https://www.engineeringtoolbox.com/