SERSO Solar energy recovery from road surfaces Conceptual design	SERSO: Energy from the road The combined solar and geothermal energy pilot project has been realized on a bridge on the Swiss highway network. The aim of the project was: to collect the heat of an asphalt bridge surface during the summer period, when     temperatures up to 60°C are reached to store it in an underground heat accumulator, and to utilize it during frost periods in winter, to heat the bridge surface, thus preventing the     formation of ice. The minimal target of this project was to guarantee equal conditions - on the bridge surface as well on the adjacent normal road surfaces. Heat pumps or any other additional heating systems are not necessary. The plant is operational since 1994.
Modes of operation	Short description of the plant Horizontal heat exchanger coils are embedded in the second layer of the asphalt surface. The coils form part of a hydraulic circuit which is filled with working fluid consisting of a glycol-water mixture. The 160 individual stainless steel coils, each 34 m long, underlie a surface area of 1300 m2 and are positioned in a plastic/cement-stabilized asphalt layer. The underground storage area is situated directly beside the bridge. The cylindrical storage volume is accessed by a number of equally spaced BHE's. The double U-tube heat exchangers form a hydraulic system with the bridge heat exchange coils. The hydraulic system consists of the connecting pipework between bridge and heat accumulator with its pumps, valves and mixing tanks. The fittings and the electronic control system are installed in a utility building beneath the bridge. The bridge heat exchange coils as well as the rock storage are sized by numerical modelling.
Energy balance	Modes of operation a) Summer operation In summer the road surface absorbs a proportion of the incident solar radiation. The heat is continuously removed by the fluid, circulating in the embedded coils and transported to the underground heat accumulator. Energy can thus be collected and transferred as long as the roadbed is warmer than the rock in the accumulator. b) Winter operation In winter the plant operates in reverse mode. Whenever danger of ice formation on the bridge is given, heat is extracted from the accumulator and transferred to the coils in the pavement. Heat transfer in this mode usually takes place when the air temperature lies between +4°C and -8°C. The control system relies on continuous temperature measurements in the roadbed, in the heat accumulator and in the connecting hydraulic system.
Initial costs	Energy balance The first year’s operation served principally to test and to stabilise the system and to gain operating experience. In the second and third years it was possible to undertake various steps towards optimising the regulating system. At the end of each complete 12 months operational cycle a substantial heat buffer remains in the rock heat store. The quantities of heat collected and lost by the bridge vary greatly from year to year, reflecting the climatic variations: hot summers in 1994 and 1995, cooler in 1996; cold winter seasons for 1994/95 and 1995/96 balanced by a mild winter for 1996/97.

Temperature changes of the rock store, energy content and seasonal runtimes.	Period	Delta T of rock store [K]	Energy content [MWh]	Runtime [h]
	Start	0.00	0.0	-
	Summer 1994	+ 4.63	132.5	1'650
	Winter 1994/95	-  3.20	41.0	1'530
	Summer 1995	+ 3.30	138.5	1'320
	Winter 1995/96	-  3.65	34.0	880
	Summer 1996	+ 2.83	115.0	580
	Winter 1996/97	-  2.05	56.0	180
	Summer 1997	+ 3.26	149.5	770
	Total	+ 5.22	149.5	-

Final remarks

Initial costs The total costs for the SERSO pilot plant amounted to aproximately 5 million CHF (3.3 million U.S. dollars). The costs include those for research, development and for the exclusively measurement campaign. A follow up plant of similar dimensions would be expected therefore to cost not more than the half of this (2.0 - 2.5 million CHF or 1.3 - 1.7 million U.S. dollars).

Initial costs	Work	costs in 1000 CHF	Proportion [%]
	Preliminary studies	957	20
	Rock store	1'380	25
	Service building	538	10
	Coils	903	10
	Measurement campaign	511	10
	Planning, supervision, reports	733	15
	Total	5'022	100

Operational costs The purely operational costs (without amortisation) are around 1’700 CHF (1’150 U.S. dollars) per year, consisting of costs for electricity for pumps. Considering a full amortisation, the costs per kWh energy (loading and/or unloading) are even cheaper than for photovoltaics. Politically economical benefits are still not considered. These should pricipally be subtracted from the inital and operational costs.

Final remarks As we have been able to show, the concept of solar energy recuperation works. Enough energy can be collected from the bridge pavement in summer to keep that same pavement ice-free during the following winter. The system does however demand a high degree of flexibility and precision from the regulation system. SERSO itself is a good example of a successful low tempeature seasonal heat store. However, the basic concept of SERSO has more to offer than maintaining a bridge free of ice. The collection of solar energy which could otherwise be radiated back to the atmosphere and it’s storage for re-use - effectively an energy recycling process - can be used to a whole range of other applications, for example: airports helicopter landing pads industrial sites sport stadiums and recreation centres carparks; roads, bridges and ramps Full reports: see Reports, books, papers.

Load pictures and plots (about 700 kB)

Update 30.03.2000/wje