Solar Process Heat Plant for Vehicle Cleaning

In 2004 the Spanish company Contank opened their new site Parking Service Castellbisbal S.A. in the town of the same name. The company is specialised in activities of the transport and logistic sector. Besides repair and paintwork, they also clean the exteriors and interiors of transport vehicles. Due to the high radiation rate of 1,471 kWh/ m2 a year, a solar process heat plant on the rooftop has been considered already from the start of the construction works. Furthermore, the integration of solar heat has been considered at the design of the cleaning processes, which ultimately made the installation a lot easier. The company Aiguasol Enginyeria conducted the planning, design, and optimisation of the solar plant’s operating method.

At the beginning of the project, the company’s heat demand has been estimated at 2 GWh/a. The cleaning process has the biggest heat demand. Here, on one hand steam is required (54 %) and hot water on the other hand (46 %). Each day ca. 70…80 m3 of water at a temperature level of up to 80 °C are required. The processes run at 5.5 days a week, respectively 264 days a year, while the site is closed on 22 days during the summer holydays. Since the year 2005 92 flat-plate collectors by the company Sonnenkraft with a light east orientation and an inclination of 25° supply a relevant part of the heat. As depicted on the image below, the solar plant with its area of 570 m2 is divided into two fields, which are located on the two rooftops of different heights. When the solar plant cannot supply enough heat with its performance of 357 kWth, the natural-gas-fuelled steam boiler is used.

The primary circuit is filled with a water-glycol-mixture (70 / 30 %) and feeds the produced solar energy to the secondary circuit via a heat exchanger. The secondary circuit contains decalcified water for the cleaning process. In order to store the solar energy, a 40 m3 pressure-less buffer tank and two small tap-water tanks of 3 m3 are deployed. After pre-heating the water is brought to the required pressure level and, if necessary, to the target temperature by the fossil fuelled steam system realized as a flow heater using a pipe-bundle-heat-exchanger. At the beginning of the project other integration concepts, which considered a direct solar steam production respectively a return heating of the heat circuit, have been discussed too. However, these concepts have been discarded in order to avoid a lower efficiency and solar yield due to the high collector temperatures.

© Contank:
About a third of the heat for hot water can be supplied by the rooftop-solar-plant, which heats up a 40 m3 buffer tank. If necessary, the solar heated water can be heated to the set-temperature by the fossil post heater using the flow heater method.

Even though the system has been simulated and optimised using TRNSYS, the actual yields were a little below the expectations, which can be explained by a too highly estimated heat demand. In order to compensate the decreased heat demand and to avoid frequent stagnation periods, a night-time-cooling has been installed additionally. Due to the wrong assumptions, the projected specific solar yield of up to 841 kWh/ m2 (429 MWh/a) could not be reached completely. However, at the same time the expected solar coverage rate of 21.5 % has been increased up to about a third. The investment costs were 270,000 €, while only the half of that had to be paid by own resources. The other half has been financed with grants and tax reliefs. Although low heat production costs of almost as little as 19 €/MWh can be achieved, the amortisation period is at ca. 9 years due to an estimated gas price of 25 €/MWh.