Determining the potential impact of DWHR technology
Drain Water Heat Recovery Methods
Different heat recovery methods
Work package 2 of the Dŵr Uisce Project aims to aid in reducing the energy consumption and CO2 emission related to space- and domestic hot water heating, in a cost efficient way. The contribution will consist of reducing the energy consumption through direct DWHR, in other words using the available heat in drain water to pre-heat the source water, and reducing the greenhouse gas intensity of thermal energy through the use of the available heat in drain water as a heat source for heat pumps.
DWHR in the residential heating market
In order to estimate the potential impact of deploying the proposed DWHR technology, its potential level of penetration in the heating market will be estimated through different scenario's. The first envisioned scenario was replacing or supplementing traditional, residential heating systems with indirect DWHR, using a heat pump to upcycle the low grade waste heat.
This showed a potential for an expected 40% of the residence’s heat consumption to be delivered by the heat recovery system. A back-up system, using traditional heating systems, remains necessary to meet all heat demand.
To estimate the impact of this system, just under 700 000 homes in Ireland were considered, this resulted in the conclusion that this system would decrease the heating related greenhouse gas emissions by up to 22%. However it would increase increase the cost of heating by around 120% for these residences. The results of this study, as a detailed methodology were published in the journal Energy & Buildings and presented at the 3rd EWaS conference.
To find more cost-effective ways of recovering this heat, two strategies can be considered. One can move upstream, closer to the heat and waste water source, this results in higher temperatures of the drain water, but lower volumes. The example of kitchen drain water will be studied. The second strategy is to move downstream, to a sewer or even wastewater treatment system level, where temperatures are lower, but volumes larger. The example of wastewater treatment plants in Ireland will be studied.
Reduction in GHG emission per unit of produced heat
Increase in heating cost per unit of produced heat
DWHR in Commercial kitchens
The recoverable heat in the drain water in commercial kitchens was estimated at 1.4 TWh per year, responsible for about 388 000 tons of Carbon equivalent emissions. A financial criterion, the Net Present Value calculated over 10 years, was used to estimate feasibility threshold values for the kitchen water consumption, varying with the heating fuel used in each premises. This analysis resulted in a total financially feasible potential for drain water heat recovery in the hospitality and food service sector in the UK of approximately 1.24 TWh/yr , leading to a reduction in GHG emissions of 344 000 tons Ce/yr . This could be achieved at a cost of approximately 2.94 p £/ kg Ce saved , over a 10-year lifetime, which was several orders of magnitude smaller than traditional renewable heat sources such as heat pumps. Commercially available drain water heat recovery systems could, when installed, exploit around 88% of current potential in drain water heat in commercial kitchens (1.40 TWh/yr ) in a manner that is financially profitable.
However, this paper assumes the availability of a 2 m vertical drop in the kitchen drain, required to install the heat exchanger, and thus achieve optimal effectiveness. In case this head is not available, the heat recovery potential would be untapped, as in approximately 160 GWh , in kitchens where the hot water consumption is too small for financial profitability. This corresponds to a reduction in GHG emissions of 44 k tons Ce/yr and leaves a gap in the market for inexpensive DWHR devices, which could exploit this potential at lower efficiencies.
The results of this study, as a detailed methodology were published in the journal - Energy and Buildings
Related publications
Spriet, J., McNabola, A. 2019. Decentralized drain water heat recovery: A probabilistic method for prediction of wastewater and heating system interaction. Energy and Buildings. Volume 183, pp. 684-696
Spriet, J., McNabola,A. 2018. Decentralized Drain Water Heat Recovery: Interaction between Wastewater and Heating Flows on a Single Residence Scale. Proceedings The 3rd EWaS International Conference on “Insights on the Water-Energy-Food Nexus”, 2, 583.
Spriet, J., McNabola, A. 2019. Decentralized drain water heat recovery from commercial kitchens in the hospitality sector. Energy and Buildings. Volume 194, pp. 247-259