Large-scale integration of wind power into different energy systems

The paper presents the ability of different energy systems and regulation strategies to integrate wind power. The ability is expressed by the following three factors: the degree of electricity excess production caused by fluctuations in wind and Combined Heat and Power (CHP) heat demands, the ability to utilise wind power to reduce CO₂ emission in the system, and the ability to benefit from exchange of electricity on the market. Energy systems and regulation strategies are analysed in the range of a wind power input from 0 to 100% of the electricity demand. Based on the Danish energy system, in which 50% of the electricity demand is produced in CHP, a number of future energy systems with CO₂ reduction potentials are analysed, i.e. systems with more CHP, systems using electricity for transportation (battery or hydrogen vehicles) and systems with fuel-cell technologies. For the present and such potential future energy systems different regulation strategies have been analysed, i.e. the inclusion of small CHP plants into the regulation task of electricity balancing and ancillary grid stability services and investments in electric heating, heat pumps and heat storage capacity. The results of the analyses make it possible to compare short-term and long-term potentials of different strategies of large-scale integration of wind power.     

Integrated energy systems and local energy markets

Significant benefits are connected with an increase in the flexibility of the Danish energy system. On the one hand, it is possible to benefit from trading electricity with neighbouring countries, and on the other, Denmark will be able to make better use of wind power and other types of renewable energy in the future. This paper presents the analysis of different ways of increasing flexibility in the Danish energy system by the use of local regulation mechanisms. This strategy is compared with the opposite extreme, i.e. trying to solve all balancing problems via electricity trade on the international market. The conclusion is that it is feasible for the Danish society to include the CHP plants in the balancing of fluctuating wind power. There are major advantages in equipping small CHP plants as well as the large CHP plants with heat pumps. By doing so, it will be possible to increase the share of wind power from the present 20 to 40% without causing significant problems of imbalance between electricity consumption and production. Investment in increased flexibility is in itself profitable. Furthermore, the feasibility of wind power is improved.     

Management of fluctuations in wind power and CHP comparing two possible Danish strategies

Both CHP (combined heat and power production) and wind power are important elements of Danish energy policy. Today, approximately 50% of both the Danish electricity and heat demand are produced in CHP and more than 15% of the electricity demand is produced by wind turbines. Both technologies are essential for the implementation of Danish climate change response objectives, and both technologies are intended for further expansion in the coming decade. Meanwhile, the integration of CHP and wind power is subject to fluctuations in electricity production. Wind turbines depend on the wind, and CHP depends on the heat demand. This article discusses and analyses two different national strategies for solving this problem. One strategy, which is the current official government policy known as the export strategy, proposes to take advantage of the Nordic and European markets for selling and buying electricity. In this case, surplus electricity from wind power and CHP simply will be sold to neighbouring countries. Another strategy, the self-supply strategy, runs the CHP units to meet both demand and the fluctuations in the wind scheduling. In this case, investments in heat storages are necessary and heat pumps have to be added to the CHP units. Based on official Danish energy policy and energy plans, this article quantifies the problem for the year 2015 in terms of the amount of surplus electricity, and investments in heat pumps, etc. needed to solve the problem are calculated. Based on these results between the two different strategies, the conclusion is that the self-supply strategy is recommended over the official export strategy.     

Modelling of energy systems with a high percentage of CHP and wind power

This paper presents the energy system analysis model EnergyPLAN, which has been used to analyse the integration of large scale wind power into the national Danish electricity system. The main purpose of the EnergyPLAN model is to design suitable national energy planning strategies by analysing the consequences of different national energy investments. The model emphasises the analysis of different regulation strategies and different market economic optimisation strategies.At present wind power supply 15% of the Danish electricity demand and ca 50% is produced in CHP (combined heat and power production). The model has been used in the work of an expert group conducted by the Danish Energy Agency for the Danish Parliament. Results are included in the paper in terms of strategies, in order to manage the integration of CHP and wind power in the future Danish energy supply in which more than 40% of the supply is expected to come from wind power.     

Modelling grid losses and the geographic distribution of electricity generation

In Denmark more than 40% of the electricity consumption is covered by geographically scattered electricity sources namely wind power and local CHP (cogeneration of heat and power) plants. This causes problems in regard to load balancing and possible grid overloads. The potential grid problems and methods for solving these are analysed in this article on the basis of energy systems analyses, geographic distribution of consumption and production and grid load-flow analyses. It is concluded that by introducing scattered load balancing using local CHP plants actively and using interruptible loads such as heat pumps, requirements of the transmission grid are lowered thereby reducing or eliminating needs of grid reinforcement. It is important that load balance is kept at local level and not just at an aggregate level.     

Contribution of wind power and CHP to exports from Western Denmark during 2000–2004

The experience of Denmark is used by the United Kingdom's anti-wind lobby to demonstrate that intermittency and inaccuracies in wind forecasting make wind power ineffective and expensive. A further assertion is that most of the power is ‘unwanted’ since up to 80% of it is exported. Here, available data for Danish energy production for 2000–2004 is used to assess the link between wind generation and exports and test the validity of these claims.Net exports in Western Denmark showed good correlation with wind production. However, they were more significantly correlated with the production from local combined heat and power (CHP) plants. In order to test the 80% export claim, a simple technique was devised to correlate and rank hourly net exports and generation from wind and local CHP. In the case where net exports were primarily attributed to (or blamed on) wind, 44–84% of annual wind production was deemed to be exported, with wind ‘causing’ 57–79% of net annual exports. For this extreme scenario, the percentage values are in line with those of critics. However, under the opposite extreme scenario in which exports are attributed to local CHP, 77–94% of exports were caused by CHP and only 4–32% of wind production was exported.Overall, this study shows that there is some degree of correlation between net exports and wind power, but that the claim that 80% is exported is unwarranted since it ignores the demonstrably stronger influence of local CHP.     

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.     

Comparative analyses of seven technologies to facilitate the integration of fluctuating renewable energy sources

An analysis of seven different technologies is presented. The technologies integrate fluctuating renewable energy sources (RES) such as wind power production into the electricity supply, and the Danish energy system is used as a case. Comprehensive hour-by-hour energy system analyses are conducted of a complete system meeting electricity, heat and transport demands, and including RES, power plants, and combined heat and power production (CHP) for district heating and transport technologies. In conclusion, the most fuel-efficient and least-cost technologies are identified through energy system and feasibility analyses. Large-scale heat pumps prove to be especially promising as they efficiently reduce the production of excess electricity. Flexible electricity demand and electric boilers are low-cost solutions, but their improvement of fuel efficiency is rather limited. Battery electric vehicles constitute the most promising transport integration technology compared with hydrogen fuel cell vehicles (HFCVs). The costs of integrating RES with electrolysers for HFCVs, CHP and micro fuel cell CHP are reduced significantly with more than 50% of RES.     

Do liberalised electricity markets help or hinder CHP and district heating? The case of the UK

This paper investigates whether and how Danish-style combined heat and power (CHP) and district heating (DH) can be implemented in the UK in the context of a liberalised electricity market. There is currently an absence, in the UK, of the Danish system of planning rules and also good tariffs for CHP electricity exports to the grid that led to the development of the Danish system of CHP and DH. However, there are some changes in UK planning practice that may help CHP and DH. These would need to be strengthened, but it is also the case that the way the liberalised electricity market operates in the UK effectively discriminates against small CHP plant selling their electricity to the grid. A Danish system of ‘aggregating’ CHP–DH plant using thermal stores could help to overcome this problem. However, an alternative strategy would be to establish feed-in tariffs for CHP units that are linked to DH modelled on the Danish ‘triple tariff’. This could help the UK's long-term objective of absorbing high levels of fluctuating renewable energy sources.     

System behaviour of compressed-air energy-storage in Denmark with a high penetration of renewable energy sources

In 2005, wind power supplied 19% of the 36 TWh annual electricity demand in Denmark, while 50% was produced at combined heat-and-power plants (CHP). The installed wind-turbine capacity in Western Denmark exceeds the local demand at certain points in time. So far, excess production has been exported to neighbouring countries. However, plans to expand wind power both in Denmark and in its neighbouring countries could restrain the export option and create transmission congestion challenges. This results in a need to increase the flexibility of the local electricity-system. Compressed-Air Energy-Storage (CAES) has been proposed as a potential solution for levelling fluctuating wind-power production and maintaining a system balance. This paper analyses the energy-balance effects of adding CAES to the Western Danish energy-system. Results show that even with an unlimited CAES plant capacity, excess power production is not eliminated because of the high percentage of CHP production. The optimal wind-power penetration for maximum CAES operation is found to be around 55%. The minimum storage size for CAES to fully eliminate condensing power plants operation in the optimized system is over 500 GWh, which corresponds to a cavern volume of around 234 Mm³ at an average pressure of 60 bar. Such a storage size would be technically and economically unfeasible. The analysis, however, did not include the potential role of a CAES plant in regulating the power services.     

A renewable energy scenario for Aalborg Municipality based on low-temperature geothermal heat, wind power and biomass

Aalborg Municipality, Denmark, wishes to investigate the possibilities of becoming independent of fossil fuels. This article describes a scenario for supplying Aalborg Municipality’s energy needs through a combination of low-temperature geothermal heat, wind power and biomass. Of particular focus in the scenario is how low-temperature geothermal heat may be utilised in district heating (DH) systems. The analyses show that it is possible to cover Aalborg Municipality’s energy needs through the use of locally available sources in combination with significant electricity savings, heat savings, reductions in industrial fuel use and savings and fuel-substitutions in the transport sector. With biomass resources being finite, the two marginal energy resources in Aalborg are geothermal heat and wind power. If geothermal heat is utilised more, wind power may be limited and vice versa. The system still relies on neighbouring areas as an electricity buffer though.     

Reduction of electricity use in Swedish industry and its impact on national power supply and European CO2 emissions

Decreased energy use is crucial for achieving sustainable energy solutions. This paper presents current and possible future electricity use in Swedish industry. Non-heavy lines of business (e.g. food, vehicles) that use one-third of the electricity in Swedish industry are analysed in detail. Most electricity is used in the support processes pumping and ventilation, and manufacturing by decomposition. Energy conservation can take place through e.g. more efficient light fittings and switching off ventilation during night and weekends. By energy-carrier switching, electricity used for heat production is replaced by e.g. fuel. Taking technically possible demand-side measures in the whole lines of business, according to energy audits in a set of factories, means a 35% demand reduction. A systems analysis of power production, trade, demand and conservation was made using the MODEST energy system optimisation model, which uses linear programming and considers the time-dependent impact on demand for days, weeks and seasons. Electricity that is replaced by district heating from a combined heat and power (CHP) plant has a dual impact on the electricity system through reduced demand and increased electricity generation. Reduced electricity consumption and enhanced cogeneration in Sweden enables increased electricity export, which displaces coal-fired condensing plants in the European electricity market and helps to reduce European CO₂ emissions. Within the European emission trading system, those electricity conservation measures should be taken that are more cost-efficient than other ways of reducing CO₂ emissions. The demand-side measures turn net electricity imports into net export and reduce annual operation costs and net CO₂ emissions due to covering Swedish electricity demand by 200 million euros and 6 Mtonne, respectively. With estimated electricity conservation in the whole of Swedish industry, net electricity exports would be larger and net CO₂ emissions would be even smaller.     

Technical aspects of status and expected future trends for wind power in Denmark

The power system of Denmark is characterized by significant incorporation of wind power. Presently, more than 20% of the annual electricity consumption is covered by electricity‐producing wind turbines. The largest increase in grid‐incorporated wind power is expected to come from large (offshore) wind farms operating as large wind power plants with ride‐through solutions, connected to the high‐voltage transmission system and providing ancillary services to the system. In Denmark there are presently two offshore wind farms connected to the transmission system: Horns Rev A (160MW rated power in the western part of the country) and Nysted (165MW rated power at Rødsand in Eastern Denmark). The construction of two more offshore wind farms, totalling 400MW by the years 2008–2010, has been announced. This article presents the status, perspectives and technical challenges for wind power in the power system from the point of view of Energinet.dk, Transmission System Operator of Denmark. Copyright © 2006 John Wiley &Sons, Ltd     

Heat accumulators in CHP networks

In a Combined Heat and Power (CHP) network, it is sometimes optimal to install a device for storing heat from one period of time to another. Several possibilities exist. If the electricity demand is high, while at the same time the district heating load is too small to take care of the heat from the CHP plant, it could be optimal to store heat from peak periods and discharge the storage under off-peak. It might also be optimal to store heat during off-peak and use it under the district heating peak load. The storage is then used for decreasing either the district heating demand or for decreasing the electricity load used for space heating. The paper shows how a mixed integer program is developed for use in the optimization process. As a case study, the CHP system of Malmö, Sweden, is used. Further, a sensitivity analysis is elaborated in order to show how the optimal solution will vary due to changes in certain input data.     

Lock-in and change: Distributed generation in Denmark in a long-term perspective

There is a renewed attention for distributed generation (DG) in European electricity sectors, but implementing DG is often problematic. This article studies the current relative success of DG in Denmark. We take into account not only recent drivers of change such as energy policy and green activism, but also long-term stability and change in the electricity supply sector. In particular we analyse the lock-in on centralized electricity supply, that still frustrates DG development elsewhere. We discuss three successive national electricity regimes, analysing regime lock-in and change in terms of technologies, actors, institutions and the position of DG. Our analysis shows that Danish energy policy as well as innovative activity by key actors indeed were crucial to the recent DG revival in Denmark. On the other hand, our long-term perspective shows that Danish energy policy and actor strategies were tuned to specifically Danish opportunities and barriers created during earlier regimes. These include experience with wind turbines and CHP as well as urban municipal and rural cooperative involvement. Copying the Danish energy policy model to other countries, regardless of national specific opportunities and barriers, will therefore not guarantee a similar outcome.     

Integration of renewable energy into the transport and electricity sectors through V2G

Large-scale sustainable energy systems will be necessary for substantial reduction of CO₂. However, large-scale implementation faces two major problems: (1) we must replace oil in the transportation sector, and (2) since today's inexpensive and abundant renewable energy resources have fluctuating output, to increase the fraction of electricity from them, we must learn to maintain a balance between demand and supply. Plug-in electric vehicles (EVs) could reduce or eliminate oil for the light vehicle fleet. Adding “vehicle-to-grid” (V2G) technology to EVs can provide storage, matching the time of generation to time of load. Two national energy systems are modelled, one for Denmark, including combined heat and power (CHP) and the other a similarly sized country without CHP (the latter being more typical of other industrialized countries). The model (EnergyPLAN) integrates energy for electricity, transport and heat, includes hourly fluctuations in human needs and the environment (wind resource and weather-driven need for heat). Four types of vehicle fleets are modelled, under levels of wind penetration varying from 0% to 100%. EVs were assumed to have high power (10 kW) connections, which provide important flexibility in time and duration of charging. We find that adding EVs and V2G to these national energy systems allows integration of much higher levels of wind electricity without excess electric production, and also greatly reduces national CO₂ emissions.     

Energy efficiency analysis and impact evaluation of the application of thermoelectric power cycle to today’s CHP systems

High efficiency thermoelectric generators (TEG) can recover waste heat from both industrial and private sectors. Thus, the development and deployment of TEG may represent one of the main drives for technological change and fuel substitution. This paper will present an analysis of system efficiency related to the integration of TEG into thermal energy systems, especially Combined Heat and Power production (CHP). Representative implementations of installing TEG in CHP plants to utilize waste heat, wherein electricity can be generated in situ as a by-product, will be described to show advantageous configurations for combustion systems. The feasible deployment of TEG in various CHP plants will be examined in terms of heat source temperature range, influences on CHP power specification and thermal environment, as well as potential benefits. The overall conversion efficiency improvements and economic benefits, together with the environmental impact of this deployment, will then be estimated. By using the Danish thermal energy system as a paradigm, this paper will consider the TEG application to district heating systems and power plants through the EnergyPLAN model, which has been created to design suitable energy strategies for the integration of electricity production into the overall energy system.     

Performance improvement of a 70 kWe natural gas combined heat and power (CHP) system

Combined heat and power is the simultaneous production of electricity and heat. CHP plants produce energy in an efficient way. A natural gas CHP system based on an internal combustion engine (ICE) is described, which has been set up at the Building Energy Research Center in Beijing, China. The system is composed of an ICE, a flue gas heat exchanger, a jacket water heat exchanger and other assistant facilities. The ICE generates power on-site, and the exhaust of the ICE is recovered by the flue gas heat exchanger, and the heat of the engine jacket is recovered by the jacket water heat exchanger to district heating system. In order to improve the performance of the system, an absorption heat pump (AHP) is adopted. The exhaust of the ICE drives the AHP to recover the sensible and latent heat step by step, and the temperature of the exhaust could be lowered to below 30 °C. In this paper, the performance of the new system were tested and compared with conventional cogeneration systems. The results show that the new CHP system could increase the heat utilization efficiency 10% compared to conventional systems in winter. All the results could be valuable references for the improvement of the CHP system.     

Area price and demand response in a market with 25% wind power

Denmark, east and west of the Great Belt are bidding areas with separate hourly area prices for the Nord Pool power exchange, covering four Nordic countries and parts of Germany. The share of wind power has now increased to 25% on an annual basis in western Denmark. This has a significant impact not only on the electricity wholesale prices, but also on the development of the market. Hourly market data are available from the website of Danish TSO from 1999. In this paper these data are analysed for the period 2004–2010. Electricity generators and customers may respond to hourly price variations, which can improve market efficiency, and a welfare gain is obtained. An important limitation for demand response is events of several consecutive hours with extreme values. The analysis in this paper is a summary and update of some of the issues covered by the EU RESPOND project. It shows that extreme events were few, and the current infrastructure and market organisation have been able to handle the amount of wind power installed so far. This recommends that geographical bidding area for the wholesale electricity market reflects external transmission constraints caused by wind power.