Imperviousness is the most critical indicator for analysing the impacts of climate change from the point of view of the urban water cycle according to Janek Laanearu, associated professor at the School of Engineering, Tallinn University of Technology.
Solutions where on-site natural resources are integrated with urban infrastructure require multifunctional optimisation for efficient usage, according to the recent article by Monika Kollo, PhD student, Tallinn University of Technology. “The integration of processes is a complicated task since rains have a random character, the heat extraction process is dependent on the available heat transfer technology, and the demand for low-temperature water varies by location. In the near future, the growing demand for energy in urban areas makes the recycling of spent energy inevitable,” she explained.
Professor Laanearu noted that thermal energy can be extracted from water systems owing to temperature differences. “Until now, low-temperature water has not been viewed as a carrier of thermal energy in the urban environment, and a lot of heat has been directed to natural water bodies such as lakes, rivers, the coastal sea in the Baltic Sea region,” he said. He also noted that we need financial approaches to make thermal energy recovery from urban water systems more attractive and competitive compared to other renewable sources.
Within the PhD project, Kollo and Laanearu are researching heat extraction from stormwater, which can be an option for producing domestic hot water during rainy season. However, as rainwater volume and temperature follow the seasonal weather pattern, there are some issues to get around in the boreal climate that has a seasonally alternating temperature.
Exact Information needed
Laanearu sees that a network of heat extraction that includes individual heat-recovery schemes has the potential to be broadly implemented in the Baltic Sea region. “Mapping local conditions is essential in finding the most suitable regions—hot spots—for the future application of heat extraction schemes by combining information about heat demands with data about available heat sources,” he said.
What is more, to recover heat efficiently, it is also essential to ensure that it is used locally, and adverse effects on the environment (pollution of lakes, rivers, coastal sea, etc.) and industrial processes (waste water treatment, etc.) are avoided.
Storage tanks are used for rainwater storage in cities. Laanearu pointed out one possible solution out of many: the water volume in storage tanks and the rainwater catchment area that corresponds to the precipitation statistics of the rainy seasons and the hot water consumptions of different types of buildings needs to be known to manage low-temperature water. “The availability of thermal energy from stormwater depends, in turn, to a great extent on the water flow in the stormwater system and heat transfer technology (e.g., heat exchanger, heat pump, etc.). For effective heat transfer from a storage tank to occur it is necessary to understand the fluid dynamics of temperature-stratified flow,” he explained.
Computational Fluid Dynamics’ Simulations give Answers
Laanearu noted that it is not possible to solve the issue only by using flow hydraulic formulae and that is why Computational Fluid Dynamics (CFD) simulations are needed to visualize buoyancy-driven flow patterns in a storage tank. “Almost all heating systems that include renewable energy heat sources have a thermal storage tank where cold and hot water are mixed together. The heat and mass transfer in thermally stratified water should be modelled to extract heat efficiently from a thermal storage tank” he emphasised.
“Preferably, heat for hot water production should be extracted from the warmer water in the interior of the tank, and cold water should be discharged. This is not easy to establish without CFD simulations,” Laanearu explained. Such problems also occur in other similar applications, e.g., where water is mixed with oil.
Options for the Future
Temperature inside a stormwater system was not a well-known subject, and it should be investigated in future works, Laanearu suggested. “Cold recovery from a storage tank is an option that could replace conventional air-air cooling systems for the cooling of buildings that consume a lot of electric energy during summer,” he added. According to Laanearu, water temperature is more stable compared to air temperature in the warm seasons, especially when peak cooling loads are required for a large commercial building such as shopping centre. Data centres are also huge electric heaters because they consume energy and all this electricity is converted to heat all year round. If we compare water cooling and air cooling applications in data centres, we can see that air cooling systems are more widespread.
Although there are more comprehensive experiences in the field of district cooling in Scandinavian countries, the first district cooling plant and network in Estonia was opened in Tartu in 2016.
 Kollo, M. & Laanearu, J. (2017) An optimal solution of thermal energy usage in the integrated system of stormwater collection and domestic-water heating. Urban Water Journal, Volume 14, pg 212-222 http://www.tandfonline.com/doi/abs/10.1080/1573062X.2015.1086006?journalCode=nurw20
 Kollo, M., Laaneru, J. & Tabri, K. (2017) Hydraulic modelling of oil spill through submerged orifices in damaged ship hulls. Ocean Engineering, Volume 130, 15 January 2017, pages 385–397. http://www.sciencedirect.com/science/article/pii/S0029801816305352
This article was funded by the European Regional Development Fund through Estonian Research Council.