Heat Harvest - Harvest of urban solar excess heat from buildings and surfaces to avoid summer overheating in cities
Goals, contents and results
In order to avoid urban heat islands, greening measures as well as green and/or water surfaces are already being propagated and used in many places. However, the use of these solutions is not always sufficient, approvable or desirable, such as in old, historic or listed buildings. A simple, invisible and seasonal solution is the "harvesting" of solar urban excess heat from building surfaces, sidewalks, roads and squares through shallow absorber ducts, which are then used in borehole heat exchanger (BHE) fields for later use as heat source for the buildings. Due to the high temperature level of urban surfaces of up to 50 ° C, inlet temperatures into the BHE storage of up to 40 ° C can be expected.
Conventional calculation and simulation methods for the design of BHE fields are limited by the conditions that occur in strongly heated inner city areas, or simulations with higher temperatures are not easily possible. The underground temperatures of a city are already increased by building and use compared to the climatic conditions. The introduction of waste heat for seasonal storage and the integration of heat pumps is therefore very sensitive and must be based on reliable and accurate figures and calculations.
Therefore, some technical questions still need to be clarified before a planned pilot project can be realized. The aim of the project Heat Harvest was to carry out comprehensive investigations of the thermal behavior of the subsurface in the case of solar thermal waste heat in a large-scale test facility on an open-air site and accompanying laboratory experiments as well as simulations under different conditions. In this way, the appropriate framework conditions for the harvest of solar waste heat in urban areas can be explored purposefully in order to later embed the approach into a larger cooperative research and eventually a demonstration project.
The results showed that with a geologically suitable underground, higher surface temperatures which occur in urban areas, can be introduced into borehole heat exchanger fields without negative thermal effects. The technology of harvesting excess solar heat in urban areas and underground thermal storage has great potential in cities that are particularly affected by the Urban Heat Island-effect.
The most important findings from the project can be summarized as follows:
- The harvesting of excess solar heat in cities can make a significant contribution to avoiding or reducing the 'Urban Heat Island' effect.
- At the same time, the concept leads to an increase in efficiency in heating operation and thus makes a contribution to the decarbonisation of space heating.
- The higher feed-in temperature (up to 40 ° C instead of the usual maximum of 30 °C) lead to a significant increase in the transmission capacity of the BHE, but not to excessive heating of the subsurface.
- The lateral radius of influence is spatially very limited with suitable geology.
- The corresponding "suitable geology" is a clay-silt-dominated system, as is typical for the Vienna urban area south of the Danube, for example.
- Seasonal heat storage in geothermal BHE fields and generally the underground is particularly interesting for densely built cities, since it is a space-saving and invisible technology.
- A precise knowledge of the subsurface (structure, groundwater, thermal conductivity, thermal capacity) is essential for correct planning and efficient operation of a geothermal storage.
- The use of excess solar heat is also of interest for (listed) existing buildings for retrofitting.
- The findings not only apply to the combination of asphalt collector and geothermal borehole heat exchangers, but also apply to all seasonal heat sources (also for classic solar thermal collectors).
- Dimensioning, operation and control of the system:
o As the storage tank is loaded, the COP of the heat pump increases on the one hand, which in turn reduces power consumption and heat extraction increases by around 20%.
o On the other hand, the increase in the storage tank temperature leads to a reduced temperature difference to the heat source asphalt collector: As a result, the heat input decreases over the years.
o At the beginning, the heat input must be significantly greater than the withdrawal, if the storage tank is fully charged, a self-regulating system is created.
o With the appropriate dimensioning, no complex control technology is required. The knowledge gained as part of this exploratory project is to be incorporated into further research projects and used in the medium term to carry out a demonstration project. Since it was a question of exploration at the current stage, the findings obtained were mainly presented in several publications and at events for the scientific research community.
In addition, stakeholders were already involved during the project implementation phase, for whom the use of this new type of technology in their work environment can be of great ecological as well as economic advantage. Since several property developers have already expressed interest in the application and are seriously considering testing and implementing it in their projects, the consortium sees considerable market potential.
The following target groups can therefore be considered relevant:
- Building operators
- Industrial and infrastructure companies with large usable areas
AIT Austrian Institute of Technology GmbH
Project or cooperation partners
- TERRA Umwelttechnik GmbH
- Geologische Bundesanstalt