Environmental Engineering Reference
ment for the fed-in electricity will show in euros and cents whether a system is
living up to the planners' promises.
The output of a solar thermal system can also be monitored with a heat volume meter.
However, in practice these are hardly ever used because of cost. In very sunny coun-
tries a solar thermal system can cover total requirements. The cost-effectiveness is
also usually high in these cases. In colder climates solar thermal systems are almost
always supplemented by conventional heating systems, which make up for what the
lack of sunshine cannot cover. No direct compensation is given for the heat output
yield of a solar thermal system. A system only pays for itself indirectly through the
savings in the fuel costs of the heating system. It is mainly the movement of fuel prices
that establishes whether a system is paying for itself and at which rate. The higher
fuel prices climb, the faster a solar thermal system will pay for itself. If, on the other
hand, fuel prices fall, the economic viability of a solar system is less favourable.
Figure 6.21 shows the payback periods for a typical solar thermal system for heating
domestic water. This is the time it takes for the fuel costs a system has saved to
break even with the investment costs. If hot water is heated electrically for around
0.2 euro/kWh, the system will be amortized very quickly. The amortization becomes
more diffi cult with particularly high-quality and expensive systems when fuel prices
are low. In this case public grant programmes could be available to improve the
economy of these systems.
investment costs in
fuel price in € /kWh
Figure 6.21 Payback periods for a solar thermal domestic hot water system with backup
heating system (without interest rate effects and price increases; fuel savings:
2000 kWh/a; annual operating costs: 2% of investment costs).
Solar thermal systems are among the most environmentally friendly renewable
energy systems available. When they are used, they normally save on fossil
fuels, such as oil and natural gas, and thereby contribute actively towards climate