Solar pump study for nature reserve

Direct-coupled solar powered pumping systems for environmentally sensitive areas are being studied as alternatives to battery-backed systems as Simon Bouwmeester, Bernard van Hemert and Mark Koerft of Ecofys report.

In 1999, Staatsbosbeheer, the Dutch National Forest Service, commissioned a redesign of the nature reserve 'De Steendert'. This nature reserve is located in the municipality Ophemert, about 7 km from the town Tiel. The reserve is about 80ha and mainly consists of wetlands, and some groups of trees. The area is provided with water from the nearby river Waal. Certain types of plants do not grow in this area because of relative drought.

The water household of the reserve is altered by constructing two 5ha ponds, one which collects the percolating water from the river Waal and the other which functions as a water storage reservoir. The second pond provides the area with sufficient water so an appropriate water level is obtained.

The water level in the percolation water pond fluctuates between +2.00m NAP (Nieuw Amsterdams Peil, the Dutch reference for altitude measurements) and +2.20 NAP, while the water level in the storage pond must fluctuate between +3.40m NAP and +3.70m NAP. A pumping system lifts the water from the lower into the higher pond.

Connecting the pumping system to the main grid requires an 1km long electrical cable, which costs about E37,000. The photovoltaic modules costs E14,000. Moreover, 'The Steendert' is a nature reserve and digging a cable into the ground had to be avoided. Thus, construction of an autonomous direct-coupled photovoltaic pumping system was seen to be both economically and technically attractive.

Expected achievements
Conventional autonomous photovoltaic systems are battery-backed to buffer electrical energy. This buffer enables the system to continue to operate during periods of reduced solar irradiation. A storage pond is advantageous in the sense that it stores energy, thus making a battery bank redundant.

The efficiency of the storage pond is higher than that of a battery bank because no energy conversion takes place. For example, a mere one centimetre increase in water level equals a whole day pumping. A battery bank is not necessary and the pump can be directly coupled to the photovoltaic array. Moreover, BOS and maintenance cost are reduced and system design is ecologically friendly by eliminating battery banks - depreciated lead-acid batteries are considered heavy poisonous chemical waste. Direct-coupled pumping systems are expected to be both economically and technically competitive with their battery-backed counter parts.

Comparison of systems
The aim of this research is to compare the technical and economic feasibility of the direct-coupled and battery-backed pumping system. The following questions concerning the feasibility will be answered:

  • Can the system maintain the water level in the storage pond between +3.40m NAP and +3.70m NAP?
  • Do the direct-coupled and battery-backed pumping systems transport an equal amount of water on a monthly basis?
  • What is the overall efficiency of both direct-coupled and battery-backed pumping systems?
  • What is the utilisability of the direct coupled system?
  • What is the estimated installation cost of the direct-coupled, battery-backed and grid connected pumping system?
  • What is the estimated maintenance cost of the direct-coupled, battery-backed and grid connected pumping system?

Ecofys supervised the construction of a dual photovoltaic pumping system by VOPO, which controls the water level in the storage pond. The system consists of two 45 m3/h pumps, one powered by an 1kWp photovoltaic array with a 230 Ah battery bank and one direct powered by an 1kWp photovoltaic array. In order to increase the life time of the battery bank a smart pump control algorithm is devised. This algorithm determines if the battery-backed pump should be switched on depending on the water level in the storage pond and state-of-charge of the battery bank.

Measurement equipment is installed in order to measure and compare the characteristics of both systems.

Overcoming lift problems
Monitoring of both systems started in May 2000. Preliminary analysis showed that the direct-coupled system starts pumping at irradiation levels of 300W/m2 and above. Available data hints that the direct-coupled system lifts a somewhat smaller amount of water than its battery-backed counterpart.

Therefore introducing a device which insures optimal matching between pump motor and solar modules, e.g. a maximum power-point tracking device, will certainly reduce any difference in performance between both systems. However, more data will be gathered and a more elaborate analysis will be performed in order to present a crisp answer to the above questions.

Direct-coupled pumping systems are expected to be both economically and technically competitive with their battery-backed counter parts.

Pumping is only necessary during the months April through August. Assuming a duty cycle of 7 solar hours per day, the pumping system is designed to transport 90m3 water per hour. The energy generated by 2.16kWp photovoltaic modules covers the energy demand of the pump during a reference year.

The aim of this research is to compare the technical and economic feasibility of the direct-coupled and battery-backed pumping system.

Data suggests that the direct-coupled pump lifts a somewhat smaller amount of water. Final analysis of the monitoring results will be available at the end of this year.



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