Vast paved surfaces in our urban environment heat up quickly on a sunny summer day. The idea of an asphalt collector is to capture this energy for the purpose of heating. Two fields of application can be destinguished. The first is the defrosting of the road surface itself during the cold season to keep the road free of ice and thereby increasing the safety for the users. The second field of application is to use the harvested energy for heating buildings.
In both cases, the collectors are connected to aquifers for inter-seasonal thermal transfer.
- sun->thermal energy
- heating energy->road surface
Description of the Process
Road Energy System is a commercially successful example of an asphalt collector by the dutch engineering and construction company Ooms Civiel in Scharwoude, The Netherlands. Research on asphalt collectors has been conducted in close cooperation with the Technical University Delft.
Ooms installed an asphalt collector on a piece of road neighboring their headquarters in Scharwoude. 2250m2 of road surface were equipped with the Road Energy System. The pipes of the heat exchanger are fixed on a grid to keep them in place and to allow the heavy road building machinery to drive over it while applying the different layers of asphalt.
A pumping station connects the collector with the thermal storage and the building. In the Scharwoude project more than 200 sensors were installed to monitor and optimize the overall performance of the system.
The underground storage is a crucial component of the system for the transfer of thermal energy between the seasons.
In Summer the building is cooled with cold water from the ground. The temperature of the water is further increased by the asphalt collector and the thermal energy is finally stored in the aquifer. An additional benefit of extracting the heat from the paved road surface is a better micro-climate with lower peak temperatures.
In winter mode, heat is extracted from the aquifer and used in the building. The temperature of the return is still high enough to be pumped through the piping embedded in the paved surfaces to keep them free of ice.
The price per square meter of an asphalt collector is relatively low, compared to other solar thermal collectors. One square meter of asphalt collector comes with a price tag of about 123€ (£100), whereas the most simple collector to be installed on a flat roof costs about 173€ (£140). For comparison, a flatbed collector can cost up to 740€ (£600) per square meter. (in 2007 by ICAX)
Result of project
The installation of an asphalt collector can result in saving up to 50% of energy for heating and cooling, equal a 50% reduction of CO2 emissions. The lifespan of the piping is quite simmilar to the normal interval of resurfacing roads. By adding piping to the upper layers of the road construction, one can benefit from solar thermal energy for in between 12 to 15 years. There is even a positive influence of the collector on the asphalt. The peak temperatures of the road surface are less extreme, protecting the asphalt from freezing in winter and from softening in summer.°°
According to the 'energy capacity' diagram of Arian de Bondt each square meter has an average heating capacity of 250W. This value is calculated with the assumption that the water pumped into the asphalt collector has a temperature of 15°C. At lower flow temperatures (5°C) the capacity is increased to 288W. Not included in this calculation is the cooling capacity when asphalt temperatures drop below the level of water temperatures. Not surprisingly, the cooling capacity increases with higher flow temperatures.
The 250W capacity per m2 resembles a total amount of energy of ~1000 kWh that could be harvested during 4000 hours.
Policy and legal context
As the idea of asphalt collectors is relatively new, there are not yet established rules for the planning and permission procedures. The overlaping resopnsibilities of different levels of public administration and the private investor, who wants to harvest thermal energy, make planning a challenging task.