امکان سنجی نیروگاه های CHP با فروشگاه های حرارتی در بازار لحظه ای آلمان

The European Energy Exchange (EEX) day ahead spot market for electricity in Germany shows significant variations in prices between peak and off-peak hours. Being able to shift electricity production from off-peak hours to peak hours improves the profit from CHP-plant operation significantly. Installing a big thermal store at a CHP-plant makes it possible to shift production of electricity and heat to hours where electricity prices are highest especially on days with low heat demand. Consequently, these conditions will have to influence the design of new CHP-plants. In this paper, the optimal size of a CHP-plant with thermal store under German spot market conditions is analyzed. As an example the possibility to install small size CHP-plant instead of only boilers at a Stadtwerke delivering 30,000 MW h-heat for district heating per year is examined using the software energyPRO. It is shown that, given the economic and technical assumptions made, a CHP-plant of 4 MW-el with a thermal store participating in the spot market will be the most feasible plant to build. A sensitivity analysis shows to which extent the optimal solution will vary by changing the key economic assumptions.

An increasing energy demand, depletion of fossil energy resources and the emission of green house gases provide incentives to develop and fully utilize highly efficient energy technologies. Cogeneration (combined heat and power, CHP) is a well known and highly efficient approach to produce electricity and heat in a single thermodynamic process [1] and [2]. By cogenerating the electricity and heat, CHP-plants have the possibility to decrease fuel consumption by 20–30% as compared to decoupled production in conventional power plants and boilers [3] and [4]. This technology reduces overall fossil fuel consumption and thus the energy is generated in a more environmentally friendly way [1], [5] and [6]. Promotion of high-efficiency cogeneration based on a useful heat demand is also required under the EU Directive 2004/8/EC [5]. Denmark is one of the countries in Europe that has been able to develop a comparatively high share of CHP production, a significant part of it being decentralized plants with an electrical output of less than 20 MW. This has been achieved mostly by granting feed-in tariffs. However, the biggest achievement is not merely the high share of CHP production, but also that those plants have been incentivized to operate flexibly by the tariff structure which has been higher during the day than during the night time and even higher during peak hours in the middle of the day. Plants could be designed for flexible operation by installing significant thermal stores. This way decentralized CHP-plants in Denmark have been rewarded when matching their production better with the electricity demand and are now well prepared for being an active participant in the power market. Having a thermal store also makes it possible to operate the production units at the most fuel efficient load and to store the surplus heat. If the prime movers are gas turbines or spark-ignited gas engines the most fuel-efficient operation is full load [7]. The advantage of the Danish operating strategy becomes even more obvious when acting under market conditions with hourly prices. The larger the difference between peak and off-peak prices the more attractive it becomes for flexible plants. In Germany, one of the largest energy markets in Europe, high-efficiency cogeneration especially in combination with district heating/cooling is regarded as strategic technology to support the government’s energy and climate policy goals. However, targets set in the CHP act of 2002 have not been achieved. In June 2008 German Parliament has approved a new CHP law, which aims at doubling the total share of CHP electricity to 25% by 2020. The estimated economic potential for CHP electricity production in Germany is lying between 300 and 350 TW h-el per year. This translates into about 35 GW-el of CHP capacity [8]. CHP electricity is promoted by bonus payments for the produced electricity paid on top of the achieved electricity sales price. Besides the bonus and the commodity price CHP operators will receive a compensation for avoided grid use if connected at a lower voltage grid. The setup results in the CHP-plant being exposed to the electricity market. Electricity spot prices in Germany show a distinct daily pattern with very high peak prices, thus the application of a flexible operating strategy can be expected to be highly attractive in the German market. Also, Germany is one of the EU leaders in renewable energies. Especially wind and solar power production is fluctuating, hence increasing the demand for flexible response from other producers. Two aspects of the German CHP market are conspicuous though: despite the support scheme decentralized CHP electricity production is still rather low, and interestingly, only few decentralized CHP-plants in Germany are designed to operate in a flexible, market oriented way. Thermal storage tanks are rarely built or relatively small. This paper shows that the German power market is highly attractive for decentralized CHP development if the plants are designed and operated as flexible units that take advantage of the fact that electricity is priced differently at different times of the day and week. This method proposes a plant design that differs significantly from common practice in Germany.

Having a thermal store, CHP-plants gain flexibility and may achieve improved economic results if managed properly. Moreover operators of CHP-plants will gain an increase in security when planning their day ahead schedules as fluctuations in the heat demand can be compensated with the store. The analyses in this paper lead to the following conclusions: – The German electricity market contains an incentive to design and operate decentralized CHP-plants flexibly. – Analyzing a German energy plant delivering 30,000 MW h-heat per year it is found that with the assumed economic conditions a 4 MW-el capacity CHP-unit with 650 m3 thermal store is feasible. – Adding a CHP-bonus for the delivered electricity improves the economic feasibility of the CHP-plant. A bonus of 15 €/MW h-el doubles the NPV and reduces simple payback time from 9 to 10 years down to 5 years. It was found that a CHP-bonus would enable the installation of bigger capacity CHP-plants. – Higher variations in electricity prices allow achieving better economic results for a CHP-plant with thermal store. – Installing a highly efficient gas engine improves the feasibility of the plant. This highly efficient CHP-unit of 5.1 MW-el capacity shows an NPV that is 1.9 times better as compared to the NPV of a 4 MW-el capacity CHP-plant. – The economics of a CHP-plant are very sensitive to fuel and electricity prices. However, as long as both electricity and natural gas prices move in the same direction and the spread stays the same, the negative impact on the NPV is limited. The applied model provides a tool to evaluate the effects of adding flexibility to a plant. However, it is still necessary to be able to capture that value during operation. So adding a thermal store and building bigger units creates opportunities on the market but also increases the operational risk. As operators of decentralized plants tend to be very risk averse this might keep them from applying these methods. In general, the feasibility of CHP-plants with thermal store depends on the individual conditions, such as electricity and fuel prices, their variation over time, investments costs, national energy policy, taxes and the local energy demand. A change in the volatility of spot prices will influence the design of CHP-plants; i.e. especially the capacities of the CHP-unit and the thermal store. High variations in spot prices provide a significant incentive for the use of thermal stores at CHP-plants. A CHP-bonus scheme may help to increase economic feasibility of CHP-plants. After all however, each individual plant has to be designed according to its specific framework conditions.