What if wastewater and rainwater flowing through city sewers could be used as a source of energy for heating and cooling buildings? This is what a team of researchers from the Technical School of Construction (ETSEM) of the Polytechnic University of Madrid has been studying, intending to understand the impact that this resource, which has so far been underused when it comes to providing energy to buildings, could have.
To do so, the UPM researchers have taken as a model an office building with a low-energy model located in Madrid and have evaluated whether or not the energy contribution of wastewater can be sufficient to warm and cool it.
"The primary objective of this research is to estimate the potential of wastewater to cover the annual thermal demand for heating and cooling. The secondary objective is to determine the impact of groundwater and rainwater on the thermal efficiency of a wastewater heat exchanger to ensure thermal comfort inside the building," explained Inmaculada Martínez, researcher at ETSEM and one of the authors of this work.
The main objective of this research is to estimate the potential of wastewater to cover the annual thermal demand for heating and cooling
For this purpose, several factors were taken into account that could influence the results, such as in which period of the year it could be more effective to use this type of energy, the impact of stormwater on the flow and temperature of the sewerage system and, consequently, how the efficiency of heat pumps is affected as a function of source side temperature, building demand and the location of the heat exchanger within the sewer system or the hypothetical heat exchange efficiency of wastewater flowing through sewer systems based on actual temperature and flow measurements.
A more effective option than geothermal systems
The results obtained by the UfM researchers show that sewage wastewater flowing at more than 5 l/s can provide sufficient thermal energy to meet the energy needs of an office building with a demand of 45 kW (60 W/m²). The efficiency, they say, is higher for heating than for cooling, although the data is good in both cases.
"The energy contribution of sewage wastewater is more favourable in the heating scenarios than in the cooling scenarios, improving the system performance by more than 22% compared to the geothermal system. Rainwater improves cooling performance by more than 14% compared to geothermal systems," the authors explain.
The research showed that the flow rate is the parameter that most directly affects the calculation of the maximum power available in the sewerage network, assuming a constant thermal jump (2 °C).
The energy contribution of sewage wastewater is more favourable in the heating scenarios than in the cooling scenarios, improving the system efficiency by more than 22% compared to the geothermal system
"The urban sewerage network guarantees sufficient flows and continuous water renewal throughout the year, so the cost-effectiveness of a heat exchanger installed in the sewerage network, properly dimensioned, would be higher than that of other geothermal systems, as well as guaranteeing the coverage of the building's energy demand with maximum performance.
In turn, the performance of the system is determined by the characteristics of the heat exchangers and the source temperature. "The wastewater in the sewerage system maintains a constant and favourable temperature range of 15°C to 25°C throughout the year, due to the continuous heat input from the domestic hot water and the geothermal tempering of the underground sewerage infrastructure, which makes it a good alternative source of energy," they add.
Data for better positioning of heat exchangers
For the researchers, the importance of this work, recently published in the journal Buildings, lies in the fact that it addresses one of the many by-products that can be obtained from the wastewater present in the surroundings of buildings: thermal energy, and provides sufficient data to decide where to place and how to size heat exchangers in sewage systems.
"The aim is to use this water as a source of renewable energy, breaking the paradigm of water as a waste to be eliminated and recognising it as a resource with great potential," explains the UPM researcher. "The results can also help to establish a predictive method or design and sizing guidelines for heat exchangers in sewer systems, as it provides sufficient data to decide whether it is convenient to place the exchanger before or after the point where rainwater is discharged, depending on the energy demand of the building (whether heating or cooling prevails). This decision must take into account urban characteristics (asphalted areas and natural green areas), climatic conditions and rainfall," he concludes.
The recovery and use of these energy resources, energy waste that comes from energy transformations in our cities and is discharged incorporated in used water to underground urban networks, is a circular and efficient action with an important potential that this study contributes to identify, manage and exploit.
