By Jan Spriet
The research, and pilot studies, performed by the Dŵr Uisce project, show that heat recovery from drain and wastewater can find its place in the future energy landscape in three different ways, under three different operating conditions:
As an energy efficiency measure.
As an efficiency boost for individual heat pump systems.
As a heat source for heating network
1. An energy efficiency measure
This form represents the use of direct DWHR, to preheat the incoming wastewater, resulting in a reduced heat and fuel consumption of traditional heating systems. This form is particularly suited for locations with high temperatures, but with the sporadic or small flow. It is characterised by a low investment cost, but also by relatively small amounts of recovered energy. This makes it particularly suitable for new installations (e.g. in new buildings or commercial kitchens), where it can be installed at a minimal added cost, but less so for retrofitting, as these marginal gains may be deemed unworthy of the trouble.
Figure 1. DWHR as an energy efficiency measure.
Figure 2. DWHR as an efficiency boost for individual heat pump systems
2. An efficiency boost for individual heat pump systems
This second form, represents the recovery of heat for individual residences, and could be particularly beneficial for systems in remote areas with low population density. It consists of using the wastewater as an additional heat source for a traditional (air--source or ground--source) heat pump system. As the wastewater has a higher temperature than these traditional sources during most of the heating season, but (for an individual residence), does not contain sufficient energy to meet all heating demand in this building, it could be used as a booster for the temperature of the heat carrier (often brine in ground source heat pumping systems). This boost in temperature would induce a boost in COP of the heat pump.
3. A heat source for a district heating network
This last form requires the availability of large volumes of wastewater, usually in centralized locations, such as large sewer collectors or wastewater treatment plants. It uses an indirect heat recovery system to provide heat to a network using heat pumps. For this form to be profitable, large quantities of wastewater are necessary to provide economies of scale. However, the centralized locations where these quantities are available are often located at a distance from consumers. Heating networks are thus required for the distribution of the heat to consumers. This can be under the form of district heating, but also the heating network in a factory or industrial plant.
In addition to this, a wastewater source heat pump could provide for a highly efficient interface between the electric and the heating network. This provides the option for increased flexibility of both networks, required with the growing importance of intermittent renewable source, such as PV and wind, but also solar-thermal heat.
Figure 3. DWHR as a heat source for heating networks.