Feasibility of CHP-plants with thermal stores in the German spot market

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.     

Cost minimization for a local utility through CHP,heat storage and load management

The energy-system optimization model MODEST is described, especially heat storage and electricity load management. Linear programming is used for minimization of capital and operation costs. MODEST may be used to find the optimal investments and when to make them. The period under study can be divided into several linked subperiods which may consist of an arbitrary number of years. MODEST is here applied to a municipal electricity and district-heating system during three five-year periods. Each year is divided into three seasons. Demand peaks, as well as weekly and diurnal variations of, for example, costs are considered. The electricity demand is divided into the three sectors households, industries, and service. The electricity demand may be reduced by energy conservation, replacement of electric heating and load management. The profitability of load management, as well as cogeneration with and without heat storage at different prices of purchased power is calculated. At traditional Swedish electricity prices, the local utility should build a woodchips-fired steam-cycle CHP (combined heat and power) plant. Consumers would find it beneficial to reduce their electricity use by conservation and switching from electric heating to oil and biofuel. If just marginal power production costs are paid, the utility should introduce biomass-fired heat-only boilers instead. Electricity conservation is smaller at these lower prices. Load management is mainly profitable at the first price scheme which includes output-power-related charges. The heat storage should be used threefold: to cover demand peaks, as well as to enable increased CHP output when it is limited by the heat demand or to run heat pumps at cheap night electricity instead of in the daytime. © 1998 John Wiley & Sons, Ltd.     

Stockholm CHP potential—An opportunity for CO2 reductions?

The potential for combined heat and power (CHP) generation in Stockholm is large and a total heat demand of about 10 TWh/year can be met in a renewed large district heating system. This model of the Stockholm district heating system shows that CHP generation can increase from 8% in 2004 to 15.5% of the total electricity generation in Sweden. Increased electricity costs in recent years have awakened an interest to invest in new electricity generation. Since renewable alternatives are favoured by green certificates, bio-fuelled CHP is most profitable at low electricity prices. Since heat demand in the district heating network sets the limit for possible electricity generation, a CHP alternative with a high electricity to heat ratio will be more profitable at when electricity prices are high. The efficient energy use in CHP has the potential to contribute to reductions in carbon dioxide emissions in Europe, when they are required and the European electricity market is working perfectly. The potential in Stockholm exceeds Sweden's undertakings under the Kyoto protocol and national reduction goals.     

Heating Characteristics and Economic Analysis of a Controllable On-Demand Heating System Based on Off-Peak Electricity Energy Storage

The working principle of a controllable on-demand heating system based on off-peak electricity energy storage(COHSBOEES) is as follows: the cheap off-peak electricity energy is converted into heat energy for storage in the evening, and the heat energy can be extracted on demand for heating during daytime peak or flat electricity periods. This technology can promote the smooth operation of the power grid, solve the problem of peak regulation for the electrical network, and promote renewable energy consumption. Based on the controllable on-demand heating strategy, a COHSBOEES for a heating area of 1000 m^2 was designed and built. Variations in the energy consumption and operating cost of the COHSBOEES in different heating situations were analyzed. The results showed that, off-peak electricity energy storage for heating was energy saving in comparison with central heating when the heating intensity of the COHSBOEES was 70 W/m^2 and the on-demand heating rate was less than 73.0%, and the off-peak electricity energy storage for heating was energy saving at any on-demand heating rate when the COHSBOEES had a heating intensity of 50 W/m^2. After the COHSBOEES has been running for three complete heating seasons, when the off-peak electricity price was 0.25 yuan/kW·h, the energy consumption cost of the COHSBOEES can be saved by 77.6% in comparison with central heating.     

Primary energy savings through thermal storage in district heating networks

District heating is an efficient way to provide heat to residential, tertiary and industrial users. Heat is often produced by CHP (combined heat and power) plants, usually designed to provide the base thermal load (40-50% of the maximum load) while the rest is provided by boilers. The use of storage tanks would permit to increase the annual operating hours of CHP: heat can be produced when the request is low (for instance during the night), stored and then used when the request is high. The use of boilers results partially reduced and the thermal load diagram is flattered. Depending on the type of CHP plant this may also affect the electricity generation. All these considerations are crucial in the free electricity market.In this paper, a multi-scale model of storage tanks is proposed. This model is particularly suitable to analyze the operation of storage systems during the heating season and to predict their effects on the primary energy consumption and cash flows. The analysis is conducted considering the Turin district heating system as case study. Results show that primary energy consumption can be reduced up to 12%, while total costs can be reduced up to about 5%.     

Cost-effective operating strategy for residential micro-combined heat and power

It is commonly assumed that dispatch of micro-combined heat and power (micro-CHP) should be heat driven, where the unit turns on when a heat load is present, and turns off or modulates when there is little or no heat demand. However, this heat led operating strategy—typical of large-scale CHP applications—may not be economically justified as scale decreases. This article investigates cost-effective operating strategies for three micro-CHP technologies; Stirling engine, gas engine, and solid oxide fuel cell (SOFC), under reasonable estimates of energy prices. The cost of meeting a typical UK residential energy demand is calculated for hypothetical heat led and electricity led operating strategies, and compared with that of an optimal strategy. Using central estimates of price parameters, and with some thermal energy storage present in the system, it is shown that the least cost operating strategy for the three technologies is to follow heat and electricity load during winter months, rather than using either heat demand or electricity demand as the only dispatch signal. Least cost operating strategy varies between technologies in summer months. In terms of environmental outcomes, the least cost operating strategy does not always result in the lowest carbon dioxide emissions. The results obtained are sensitive to electricity buy-back rate.     

Stochastic heat storage problem—solved by the progressive hedging algorithm

In the present study, a problem concerning operation of a heat storage tank in connection with a combined heat and power (CHP) plant is considered. The heat storage is used to supply the district heating system when the CHP plant is producing electric power alone and also to redistribute optimally over time the required heat production. Stochasticity is assumed attached to the future power production, and it is assumed that accurate predictions of the future heat consumption are available. A receding horizon idea is used. The problem is solved by the Progressive Hedging Algorithm (PHA), a recently developed method to deal with multi-period optimization problems under uncertainty. The application of the method is explained in detail.     

基于负荷侧响应的含储热热电联产的风电消纳模型

为了有效减少弃风,提高风电消纳能力,该文从负荷侧出发,通过峰谷分时电价策略引导用户的用电方式,达到削峰填谷,优化负荷曲线的目的。同时,在传统热电联产机组中应用大容量储热装置,通过对储热环节的控制,解耦“以热定电”约束,提高系统调节能力。以系统煤耗量最低为目标,构建包含储热的热电联产机组与风电联合出力优化调度模型。该模型考虑系统中的含储热热电联产机组运行成本,同时兼顾储热、负荷侧响应与热电平衡的相关约束等因素,采用基于模拟退火的粒子群算法对模型进行求解,并利用算例比较不同模式下的结果,验证了模型的有效性。     

A new integrated system with cooling storage in soil and ground-coupled heat pump

For the purpose of decreasing the peak electricity, balancing the on and off-peak electric load and utilizing the renewable geothermal energy, a new integrated system with cooling storage in soil and a ground-coupled heat pump is presented. In the integrated system, the moist soil acts as the material for cooling storage, and pipes serve as the cooling storage devices and geothermal heat exchangers simultaneously. In the cooling season, the cooling energy is stored in soil during the off-peak period and is extracted for space cooling during the on-peak period. While in other seasons, the system works as a ground-coupled heat pump for heating or cooling. A mathematical model which describes the charging and discharging processes of the integrated system has been developed and validated, and a computer code has been implemented to simulate the operational performance of cooling charging and discharging in soil. A parametric study indicates that the charging inlet temperature, tube diameter, moisture content of soil and pipe distance are important factors in determining the cyclic performance of the integrated system.     

Risk-based performance of power-to-gas storage technology integrated with energy hub system regarding downside risk constrained approach

In modern power systems, the reliability of energy supplies is a real challenge for the operators. The emergence of renewable energy resources, along with multi-career users, requires multi-career systems. In this regard, the energy hub (EH) as an integrated system can be used to increase the reliability of the system. The power-to-gas (P2G) and P2G storage are two practical technologies to achieve high efficiency in energy systems. In this paper, the contribution is optimal scheduling of stochastic problem in EH system amalgamated with CHP unit, P2G storage, thermal storage, boiler, wind power, and electrical storage to supply the heat, gas, and power loads by regarding demand response program (DRP). For the electrical loads, the load shifting strategy is considered to minimize the operational cost of the EH system. In order to manage related uncertainties about electricity price, wind power, and electrical loads, the downside risk constraint (DRC) method is applied to investigate the EH system function. According to the obtained results, by increasing approximately 2.8% of the operational cost, the risk level can be reduced remarkably. And also, almost 10% of the energy shifted from peak hours to the off-peak time after DRP is applied.     

Development and assessment of geothermal-based underground pumped hydroenergy storage system integrated with organic Rankine cycle and district heating

This study develops a geothermal-based underground pumped hydroenergy storage system (UPHES) integrated with an organic Rankine cycle (ORC) and district heating. The ORC has been integrated into the system for contributing the power demand of the pump used during the charging period to pump the water from the underground to the upper reservoir. District heating has been performed by geothermal. Heat recovered from the ORC has been used for the preheating of water used in district heating before entering the upper reservoir. With this proposed integrated system, both the peak energy demand has been shifted to off-peak hours and district heating has been performed. An ORC has also been included in the system to contribute to the energy demand of the UPHES pump. In order to evaluate the system's performance, thermodynamic analysis has been performed base on energetic and exergetic efficiencies. In the base case study, 60 MWh of electricity has stored by the UPHES. Also, a 2.8 MW of heating demand of 100 houses has been met and a 1.7 MW of power has been generated by the ORC. The power of the UPHES pump has been calculated as 6.81 MW. The efficiency of the UPHES system is 73.34%. Overall energy and exergy efficiencies of the proposed integrated system have been calculated as 93.09% and 78.37%, respectively.     

A dynamic risk aversion model for virtual energy plant considering uncertainties and demand response

To full use clean energy to meet load demand of electrical and thermal, the paper proposed a novel concept of virtual energy plant (VEP) including wind power plant (WPP), photovoltaic power generation (PV), combined heat and power generation (CHP), solar collectors (SC), electric boiler (EB), heat storage tank (HSK), and incentive‐based demand response (IBDR). Firstly, the basic structure of VEP is designed, including three subsystems, namely, electricity, heating, and energy storage. Then, a basic scheduling model is constructed under the objective of maximizing operating revenue without considering uncertainty. Thirdly, the conditional value at risk (CVaR) method and the robust optimization theory are used to handle the uncertainty factors in objective functions and condition constraints, and the risk aversion scheduling model is proposed. Finally, industrial park group in northern China are chosen, for example, analysis results show (1) VEP could convert the abandoned clean energy, use HSK to store heating energy during the valley load period, and supply heating energy in the peak period to obtain the excess economic benefits. (2) Lower‐prediction accuracy will amplify the uncertainty risk, when the robust β∈[0.8,0.825]&(0.925,1], the increase of confidence level β will lead to larger increase in CVaR. Especially when β∈(0.925,1), decision makers are extremely disgusted with the risks brought by the uncertainty factors, and correspondingly, the output of clean energy becomes minimum. (3) When the capacity ratio of HSK, EB and the electricity price of peak, valley are lower than 3, the values of revenue, VaR and CVaR change faster, but the ratios are larger than 3, the values change slower, which indicates that the scale of HSK capacity needs to be properly controlled to optimize the use of clean energy, and price‐based demand response could improve the operation profit while controlling risk properly. In general, the proposed scheduling model can maximize the use of clean energy to obtain economic benefits while rationally controlling risks.     

Role of hydrogen in future North European power system in 2060

The Balmorel model has been used to calculate the economic optimal energy system configuration for the Scandinavian countries and Germany in 2060 assuming a nearly 100% coverage of the energy demands in the power, heat and transport sector with renewable energy sources. Different assumptions about the future success of fuel cell technologies have been investigated as well as different electricity and heat demand assumptions. The variability of wind power production was handled by varying the hydropower production and the production on CHP plants using biomass, by power transmission, by varying the heat production in heat pumps and electric heat boilers, and by varying the production of hydrogen in electrolysis plants in combination with hydrogen storage. Investment in hydrogen storage capacity corresponded to 1.2% of annual wind power production in the scenarios without a hydrogen demand from the transport sector, and approximately 4% in the scenarios with a hydrogen demand from the transport sector. Even the scenarios without a demand for hydrogen from the transport sector saw investments in hydrogen storage due to the need for flexibility provided by the ability to store hydrogen. The storage capacities of the electricity storages provided by plug-in hybrid electric vehicles were too small to make hydrogen storage superfluous.     

Modeling the impact of integrating solar thermal systems and heat pumps for domestic hot water in electric systems – The case study of Corvo Island

The use of solar thermal systems with electricity backup and heat pumps as hot water suppliers in residential buildings seems to be a very promising way to increase energy efficiency. Nevertheless, the massive adoption of such solutions in small networks (neighborhood, village) may induce problems in the electric grid management. This study explores the impact of such systems in small electric grids, using an hourly electricity backup load model. To test and validate the model, we used the island of Corvo (Azores), a small isolated community where it is being implemented a project of electrification of domestic hot water systems (DHW). We consider different load scenarios to manage the backup of DHW systems and analyze its consequences on the peak load and overall energy demand. For Corvo, for the best case where the backup is limited and distributed along off-peak hours, we observed an increase of 24% in the peak load and 7.5% in the annual energy demand. Critical values of peak load are found in winter, when daily solar irradiation is lower than 2000 Wh/m²/day. We conclude that the solar thermal systems are responsible for most of the peak load increase, but since they have the flexibility to adjust the electric backup hours due to the thermal storage capacity, the use of these systems can minimize the impact on the grid. Heat pumps on the other hand, albeit being more efficient in terms electric backup, are less flexible to contribute to the grid management as they operate continuously.     

Conditions for aggregation of CHP plants in the UK electricity market and exploration of plant size

Combined heat and power (CHP) plants with thermal stores may be suitable for sustainable energy production and the accommodation of fluctuating renewable energy sources. At the moment, in the UK, only a few CHP plants have thermal stores. Previous research has shown that thermal stores can improve the economics of CHP plants in the UK under the current market conditions. However, currently, it is only beneficial for CHP plants to sell their electricity to a third party, a Licensed Electricity Supplier, rather than to sell it directly to the power exchange market at prices which are much higher. If CHP plants aggregate, direct access to the power exchange market can become economically viable hence there is the possibility that thermal stores could further improve the economics of CHP plants under an aggregated electricity dispatch. This work firstly explains the conditions under which such plants could aggregate and act as a large power plant in the UK market, and secondly explores the most economic-size of gas engine and thermal store, in the case of aggregation, using energyPRO software and Excel spreadsheets. The work suggests that direct access to the power exchange market can improve the economics of the CHP plants. The highest Net Present Value (NPV), without heat dissipation, for a CHP plant exporting its electricity to the grid for a community heating load of 20 GW h, is more than £5 m, and is obtained for a 6 MW engine with a 28.2 MW h (900 m³) thermal store. The research suggests that such high electricity prices could make even larger plants more profitable than that; however, this can happen only if some of the produced heat is dissipated.     

Exploration of economical sizing of gas engine and thermal store for combined heat and power plants in the UK

There has been discussion about the extent to which combined heat and power (CHP) plants with thermal stores are suitable for sustainable energy production. At the moment, in the UK the development of this type of plant is limited. This paper analyses the economics and optimum size of CHP operating with gas engines and thermal stores in British market conditions. This is achieved using energyPRO software. It is shown that, due to the big differences in electricity prices between day and night, the use of thermal stores could be profitable in the UK. The economical size of CHP plant for a district or community heating load of 20,000 MWh per year is found to be a 3 MWe gas engine with a 7.8 MWh thermal store. In this case the analysis reveals that the use of a thermal store more than doubles the return on investments (as measured in net present value) compared with the same size of a plant without a thermal store. It is concluded that thermal stores can improve the overall economics of CHP plants in present British circumstances.     

Case analyses of heat trading between buildings connected by a district heating network

This study examined the decentralisation of energy production with possibilities of trading not only electricity but also heat. This was carried out with a limited population of buildings connected to the district heating network in a small town in Finland.

The study examined whether a buildings community could be self sufficient with respect to heat by using small CHP plants, and explored the consequences of such an energy system. It also aimed to find an optimal solution. Therefore, it searched in what type of buildings should the micro-CHP plants be installed and with what kind of strategy should they be operated in order for the community to be energetically self sufficient without producing lots of excess heat.     

Coproduction of district heat and electricity or biomotor fuels

The operation of a district heating system depends on the heat load demand, which varies throughout the year. In this paper, we analyze the coproduction of district heat and electricity or biomotor fuels. We demonstrate how three different taxation scenarios and two crude oil price levels influence the selection of production units to minimize the district heat production cost and calculate the resulting primary energy use. Our analysis is based on the annual measured heat load of a district heating system. The minimum-cost district heat production system comprises different production units that meet the district heat demand and simultaneously minimize the district heat production cost. First, we optimize the cost of a district heat production system based on the cogeneration of electricity and heat with and without biomass integrated gasification combined-cycle technology. We considered cogenerated electricity as a byproduct with the value of that produced by a condensing power plant. Next, we integrate and optimize different biomotor fuel production units into the district heat production system by considering biomotor fuels as byproducts that can substitute for fossil motor fuels. We demonstrate that in district heating systems, the strengthening of environmental taxation reduces the dependence on fossil fuels. However, increases in environmental taxation and the crude oil price do not necessarily influence the production cost of district heat as long as biomass price is not driven by policy measures. Biomotor fuel production in a district heating system is typically not cost-efficient. The biomotor fuels produced from the district heating system have to compete with those from standalone biomotor fuel plants and also with its fossil-based counterparts. This is also true for high oil prices. A carbon tax on fossil CO₂ emissions based on social cost damage will increase the competitiveness of biomass-based combined heat and power plants, especially for BIGCC technology with its high electricity-to-heat ratio.     

A comprehensive thermodynamic analysis of load‐flexible CHP plants using district heating network

Because of the rapid expansion of intermittent renewable energy, conventional coal‐fired power plants, including combined heat and power (CHP) plants, are required to improve the quick‐response ability to respond the changing demand of the grid. However, the flexibility of CHP plants is not easy to be improved because of the restriction of traditional load variation mechanism. This work presents a comprehensive thermodynamic analysis on the flexibility‐improving scheme using the thermal energy storage (TES) capacity of district heating (DH) network. A typical CHP plant and related DH network were selected as a case study. The flexibility demand under the context of renewables accommodation in the short timescale (counted by minutes) and the operational characteristics of CHP plants were analyzed on the basis of experimental data and thermodynamics. Besides, the influence of heat supply adjustment on heat users' indoor temperature was quantified with a dynamic model, and the thermal inertia of the DH network is discussed. Moreover, a thermodynamic model for the load variation processes simplified with operational characteristics was established to analyze the response ability improvement of CHP plants. Results of the case study show, the scheme can shorten approximately 34% of the response time while almost have no influence on the indoor temperature of heat users.     

Optimal operation of coproduction with storage

We discuss the optimal operation of a combined heat and power (CHP) plant with heat storage. The dynamics of the district heating (DH) network are also considered. The decision variables include production of heat and power, supply temperature and operation of heat storage. Heat demand for the DH system and shadow prices for the electrical power system are inputs to the production system. The optimization criterion is minimization of total costs over the planning period. A non-linear optimization model based on real data is formulated and representative case studies are performed.