The mobility of a solar thermal plant depends partly on tank size. The tank sizing for Case 2 (600m3) starts to be over the limit of going from factory built to site built, at least as a single tank. Up to a certain size, however, this can be arranged with 2-3 smaller tanks. Based on a telephone interview with a storage tank manufacturer this solution may also be cheaper up to 500-700 m3.

The cost of moving the panel array is unknown. Nobody has reportedly done this or documented the costs of doing it. The collector manufacturers for large arrays do not give out the information of their installation costs nor do they deliver collectors only; they sell only whole solar loop installations.

Some assumptions can be made based on a few documented projects. One german project reports [13] panel array installation costs to 60 EUR/m2 (although crane and transport not separated). Assuming the same 60 EUR/m2 for re-installation, and that dismantling and transport would be half of that, we come

to a rough estimation of 90 EUR/m2 which is about 20-25% of the initial investment. Making an estimation from the feasibility graphs for Case 1 and Case 2 this means roughly 3-6 years of prolonged payback time. It seems that with the presented feasible payback periods and current energy prices there is no room for added costs from moving the solar plant within its lifetime.

3. Conclusion

A feasibility study to find out general boundary conditions for combining solar thermal plants with small scale district heating networks under north European conditions was done. Even though the scenario was such that we connect solar to an existing plant, the conclusions made based on the results should be valid also for new plants with reasonable accuracy.

Cost breakdown:

The most costly part of the system is the collector array. The fixed costs play a smaller and smaller part in the total cost with increased system size, resulting in lower specific costs and thus improved economic viability, assuming the saved fuels are the same. The tank is the second largest investment. Other costs are relatively small compared to these, and only marginally affect the economic viability.

Pellets as fuel:

In the considered range 0.5-2 MWth, if the load is mainly space heating (Case 1) the results would imply that with todays pellet prices it will be difficult to find a feasible payback period during the plants estimated lifetime in the studied climate. Even with annual price increase rates of 5-10 % the investment seems difficult to justify economically without subsidies. For Case 2 we can find feasible payback periods shorter than the estimated lifetime of the system, but based on the studied plant portfolio, this kind of load profiles seem to be exceptions.

Oil as fuel:

If the replaced fuel is oil, then the feasible payback periods look different from the case of pellets. For Case 1 they are between 10-20 years and for Case 2 between 7-15 years depending on country and scenario.

Plant mobility:

The rough estimation made about dismantling and moving a plant implies that with current fuel prices the plant payback period is prolonged by 3-6 years. This cannot be justified regarding the feasible payback periods of the whole plant in general.

Acknowledgements

We are grateful to VAPO Oy for collaboration and financing.

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