Mapping of Cities Wastewater Heating/Cooling Capacity

Abstract

This paper is not a research paper, but is provided as support information for an oral conference presentation. It is intended to provide background information to the topic of using wastewater as a thermal source for heating & cooling buildings and describe innovative work which has been undertaken in New Zealand in mapping the distribution and capacity of thermal energy within wastewater networks. This mapping work is also applicable in Central European cities and so relevant to Sustainable Building. Approximately 20%-30% of all the energy used in New Zealand households is used in domestic hot water systems. This heat energy then flows 'down the drain' and is collected within a city's wastewater network. So flowing through cities' sewers, wastewater is a hidden and seldom used source of thermal energy, contained within infrastructure which is already existing. Its flow is constant irrespective of the seasons, and its availability is independent of wind or sun. There is a very large thermal capacity available, which is currently largely unused. Wastewater can be used for heating and also for cooling of commercial buildings, apartment buildings or district energy schemes. This can allow buildings to stop using fossil fuels and the good thermal characteristics of wastewater offer significant increases in the electrical efficiency of building heating & air conditioning systems, with associated savings in potable water when wet cooling towers are eliminated. The large thermal capacity available within the wastewater network can be modelled and mapped, to allow cities to start planning to take advantage of this resource and make significant gains in efficiency. The oral presentation will show examples of cities who have had wastewater network energy mapping done, and a range of case studies of how wastewater heating & cooling energy is being used. An example of the outcomes from these mapping studies from Christchurch City in New Zealand (population 400,000) showed that there is sufficient heat available from the wastewater network to heat approximately 10,000 houses in the city.