The results obtained with primary energy or energy-cost minimisation do not differ fundamentally, assuming the present power export tariff. In other words, it makes no sense with regard to energy and finances to run the rCHP with the unique goal of power genera­tion if there is no need for heating, since the electrical efficiency of rCHP is too low in comparison to larger power plants. This statement remains valid for systems with and without a thermal solar system. In the past, some complaints arose: the current bonus amount is claimed to be to low to ever reach break-even for a plant and should be increased. Simulation has shown that if the bonus is increased above the point where the sole export of electricity makes the system financially profitable (i. e. without heat use), then the environmental benefits of the rCHP vanish and the operation time of the solar thermal system drops dramatically. The rCHP is then operated continuously up to the point when the storage tank is full and the plant must be switched off for safety reasons. This threshold value depends upon the fuel price, power remuneration rate and the electrical efficiency of the rCHP.

As a conclusion, the power export bonus must be kept below this threshold. If this is not enough to make rCHP financially viable, then the government should offer investment subsidies (i. e. lending at a lower interest rate than the average bank rate). In any case, the incentive for a CHP plant has no influence on the decision to buy a solar thermal system or not.

If one wants to have rCHP and solar thermal systems working together, an easy solution is a seasonally dependent CHP bonus. This means that the bonus is set to zero during the period with high solar irradiation. During the space-heating period, the bonus is increased to a higher value than the constant value in order to compensate for the financial losses in the summer. Over a one-year period, the operator must end up with the same total for both bonus models. This paper specifically handles the case of the German official heating period (from September to the end of May).

The seasonally dependent bonus approach has been illustrated in Fig. 4. There, the bonus has been set above the threshold (in that case around 11 cents/kWh). In the event of a constant bonus, the solar yield of a cost-driven system drops by 60% in comparison to a system optimised for primary energy. If the bonus is only paid in the heating period, the solar yield of the cost-driven system falls only by 26%. At the same time, the operator does not suffers from any financial drawbacks since the total annual bonus is nearly the same.

In addition to better solar performance, the simulation results show that the primary energy consumption of the cost-led system is almost the same as that of a primary energy-led system (mostly thanks to the solar input). As a conclusion, a seasonally dependent bonus is not a tool to promote solar energy (the financial profit remains the same) but it prevents the displacement of solar systems by fossil-fuelled rCHP systems. It is advised to let the rCHP operator have the choice between the two bonus approaches. Thus, he can freely decide whether to purchase a solar thermal system or not.

A further case is a time-dependent remuneration model, where the grid operator varies the value of remuneration according to his needs for CHP power. Fig. 5 deals with that issue.

It has been assumed that the remuneration rate varies according to the price fluctuation at the Leipzig power exchange (LPX). Here again, the seasonally dependent bonus smoothes the displacement effect (from -43% to -19%). As a conclusion, the seasonally

dependent bonus model can partly redress the negative effect of time-variable remuneration on the solar yield.

Conclusion

The results of an investigation about the combination of solar thermal systems with rCHP in terms of energy and economy benefits have been summarised in this paper. The discussion is based on simulations of technical building services. The results gained show that under the current regulatory and institutional framework (that means that the bonus remains at current level), solar yields are not jeopardised by the CHP bonus concept. In the event, that the public grid relies more and more on rCHP power, there is a risk that solar systems are displaced by rCHP systems, which are commonly operated with fossil fuels.

A new concept that relies on a seasonally dependent bonus has been presented. Simulation results show that this concept can address the issue above.

Moreover, this study has shown the need for new intelligent control strategies for rCHP plants, that react to weather forecasts and heat and power load predictions and can consider the interaction between a building and the heat supply chain in order to optimise the yield of the plant in a such way that fossil fuel can be saved or energy cost are minimised.

Acknowledgements

This work was supported by the German Ministry of the Environment, Nature Conservation and Nuclear Safety (BMU) in the framework of the Future Investment Programme (ZIP) and the research project “Umweltauswirkungen, Rahmenbedingungen und Markt — potenziale des dezentralen Einsatzes stationarer Brennstoffzellen” (Environmental effects, boundary conditions and market potential of distributed stationary fuel cells). Other project partners are DLR Stuttgart, IFEU Heidelberg, Wuppertal Institut, Ruhr-Universitat Bochum and ZSW Baden-Wurttemberg. The report of this study has been published as a book ([Krewitt et al. 2004]).

a> _ro о (Я "го E u a)

(Winter) (Winter) (Winter) CHP bonus (cents/kWh)

800 ? 700 Ш 600 IS c о 500 Q_ X 400 300 « 200 c c < 100

Fig. 4: Thermal solar yield versus the rate of the CHP bonus in the case of a low-energy house. On the right-hand side, in comparison to a constant bonus of 12 cents/kWh througout the year, the CHP bonus has been set to zero during the summer and has been varied between 12 and 14 cents/kWh in the heating period. In this way, the performance of the solar thermal system could be improved and the total annual bonus remains almost unchanged.

Solar yield [kWh/a] or CHP bonus [EUR/a]

32000

— 43%

31500 S’ 31000 30500 ra E 30000 U) 29500 S’ 29000 o. 28500

primary energy led, energy cost led, constant energy cost led, seasonally constant bonus bonus dependent bonus

28000

Fig. 5: Variation of solar yield, total annual CHP bonus and primary energy consumption in the case of time-dependent remuneration. In the case of seasonally dependent

— 19%

1600 1400 1200 1000 800 600 400 200 0

□ Solar yield [kWh/a] □ Total annual CHP bonus [EUR/a] □ Primary energy [kWh/a]

□ Solar yield with constant bonus □ Solar yield with seasonally dependent bonus □ Total annual CHP bonus

0

900

bonus, the CHP bonus has been set to 0 cents/kWh during the summer and to 7 cents/kWh for the remaining part of the year

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