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Integrating Heat and Power Systems- Towards 100% Renewable Energy

Written on: Friday January 18th, 2008

For the past thirty years the staff at the Nordic Folkecenter for Renewable Energy (NFC) have worked at constantly pushing the limits regarding the integration and adoption of renewable energy. Their achievements have continued to show the public what can be done with limited funds and unlimited ingenuity. Recently, in order to fully utilize renewable energy generated on site, the NFC heating system was upgraded in order to show how wind, wood pellet biomass and local plant oil could be used to produce an integrated system unmatched in Denmark. The system incorporates multiple energy mediums to replace what was once supplied by conventional fuel oil. The new system not only provides a clean sustainable method for heating NFC well into the future, it provides a path that Denmark?s district heating systems, wind power producers and combined heat and power (CHP) operators can follow on a larger scale.
The director of the Nordic Folkecenter for Renewable Energy and designer of both the original and upgraded heating system is Preben Maegaard. The original system, installed 30 years ago, consisted of a fuel oil boiler (the most efficient available at that time), and solar thermal connected on a small district heating loop to provide thermal energy to four buildings. During the summer months an assortment of solar collectors on display supply the majority of NFC's heat by capturing low grade solar energy for domestic hot water. As winter approaches, the solar fraction becomes almost non existent and the oil boiler had to be started in order to provide for heating and domestic hot water. When the original system was built in the 1970?s Maegaard knew that he would one day use the wind as a heating source and installed additional lines and a 10,000 liter heat storage tank for future use. The two primary grid-tied wind turbines connected to this system are a 7.5 kW and a 75 kW machine. The wind turbines provide all of NFC's power needs on an annualized basis and they produce in excess of 150,000 kWhrs per year (which is equivalent to 15,000 kg of oil). However, as the wind is not always blowing there are times when NFC has to purchase electricity from the grid.

Energy price of various fuels in Denmark, Price per kWhr - CAD

Wind power sold to the Grid - $ 0.04
Power Bought from the Grid - $ 0.34
Fuel Oil for heating - $ 0.22
Cost of Pellets - $ 0.09

Maegaard quickly recognized that he was paying far more to buy power from the grid and to purchase fuel oil (on a per kWhr basis) then he was receiving from selling excess electricity. He starting thinking about ways to not only reduce the cost of heating but how to provide more value for the wind energy that was being produced. As seen in Table 1, wind energy sold from NFC turbines receives a price of 0.22 DKK/kWhr, while the cost for fuel oil is five times more (1.10 DKK/kWhr). Therefore, just by using the wind electricity for heating (and offsetting fuel oil costs) NFC can quadruple the value of the wind energy. Additionally, by using wood pellets in conjunction with the wind power for heat NFC has a storable energy reserve for times when there is insufficient wind. The cost of pellets is less than half the cost of the fuel oil at 0.45 DKK/kWhr. In order to complete the system and provide self reliance to NFC a combined heat and power unit was needed. This unit can provide electricity in times when there is no wind (and eliminates the need to purchase power from the grid) and the heat generated is captured and used in the district heating system. A new control strategy now allocates the amount of wind power sent to the three options in this order of preference: (i) electricity to supply NFC's electrical needs, (ii) electricity for water heating, and if there is still surplus, (iii) electricity is sold to the grid. The final upgraded system design that was implemented consisted of a wind boiler, plant oil CHP unit and wood pellet boiler in conjunction with the existing solar thermal and a 10,000 liter thermal storage tank.

A little Side Note- Combined Heat & Power and District Heating in Denmark
Combined Heat and Power (CHP) schemes generally consists of an internal combustion engine or combined-cycle to produce and capture both heat and power. Numerous fuels are used: fossil-based, renewables (i.e. biogas) or solid municipal waste. The electricity is sold to the power grid and the heat is distributed to a community through a network of underground water pipes, called district heating. Heating Plants usually consist of a boiler (but can also use waste heat or geothermal) and provide district heating, but do not generate electricity. Fuels commonly used in Danish heating plants include biomass (such as straw and wood chips) and solid municipal waste.
In 1987 Denmark created a political framework that supported the establishment of local consumer-owned and municipality-owned CHP plants in that country. The ownership of power production then gradually shifted from centralized power production to local, independent, not-for-profit energy supply. The majority of district heating loops in Denmark were installed from 1990-2001 and they were predominantly owned by the members of the community which they were supplying, anywhere from 500 to 70,000 people. Smaller centers built CHP natural gas engines, small biomass combustors or heating plants, while the larger towns and cities employed gas turbines, or a combination of all of the technologies. Systems were designed based on the fuel available, the geography and the needs of the community.
This change to decentralized CHP happened in parallel with the addition of 3,200 MW of new wind power with 85% owned and used-by-community power co-operatives referred to as Independent Power Producers (IPPs). By 2001 a total of 45% of the 35 TWh of power used in Denmark was being produced by IPPs. Of the 45%, wind power accounted for 20% and CHP 25%. As a consequence the central power utilities (Vattenfall, DONG Energy and EON) had their share of the power market reduced to 55%. This transition took only 10 years to dramatically shift almost 50% of the power production from inefficient, centralized, fossil fuel companies to local, municipal or consumer-owned companies. Coincidently this is the amount of time it takes to build one nuclear power plant, or roughly 1200 MW. This transition represented the single most important initiative to reduce CO2 emissions in Denmark.
Advantages of community-based CHP units are vast, the main benefits being that stationary Natural Gas CHP boasts an electrical efficiency of 41% compared to the average efficiency of a thermal coal plant of 30%. With heat recovery of the jacket water, exhaust, lube oil and turbo charger an overall thermal efficiency of over 85% can be achieved. This reduces both power and heating costs. In 2001 Denmark had the third lowest power prices (without taxes) in Europe with Sweden and Finland being lower due to high contributions from hydro. In contrast France, with 80% of its power coming from atomic energy, had a higher power price than Denmark. Source.

Upscalling the System Concept
In addition to providing cost effective heating for NFC, Maegaard had a secondary agenda. It is well known that Denmark's 5000 wind turbines produce approximately 20% of the country electrical load on an annualized basis. During times of high wind speeds the wind turbines can produce over 100% of the national consumption. Since the centralized coal plants, provide base load and have limited turndown capability, a significant amount of electricity at very low spot market prices gets shipped off to Norway, Germany and Sweden. To compound this, numerous community-based CHP plants may continue running (and generating even more electricity onto the grid) to ensure adequate heat supply to their consumers. In fact, sometimes Denmark has to pay to get rid of their excess electricity. This is one reason Denmark has capped the national installed wind capacity and is currently not allowing increased growth.
The new NFC heating system has been built to mimic, on a small scale, the Danish power and district heating systems. The district heating systems distributed across Denmark typically use CHP schemes (some are heating plants only) and use various fuel sources including wood chips, straw, biogas, solid municipal waste, natural gas, coal and others. These fuels are essentially energy storage mediums comparable to the NFC wood pellets. The NFC system only uses wood pellets (for heating) and CHP (for heating and power) when renewable electricity and solar thermal cannot fulfill the heat and/or power requirements.
With his system up and running, Maegaard is promoting that the concept be up-scaled to a regional and even national level based on integrating the existing community-based CHP and district heating plants with electrical heating using excess renewable electricity. Once a secondary electric heating element is installed, these facilities can switch to electric heating in times of excess wind capacity. As the wind slows down and the supply spike dissipates, the heating plants will be required to restart their primary fuel systems in order to continue uninterrupted service. Since most district heating systems have relatively quick startup times as compared to coal or nuclear power plants, using the CHP and district heat systems as swing producers makes sense.
The basic capital cost of such a scheme would include the costs of installing secondary electric heating elements at all heating plants in addition to a central control system with real time production data for wind power, CHP & heating plant loads, and national grid demand. It is anticipated that this would make economic sense, due to the low/negative price being received for power exports, although detailed economics have not been performed. Equally important is that this concept allows Denmark to increase the overall contribution of renewably-generated electricity, increased energy independence and further reduce CO2 emissions. In addition, many of the existing CHP and heating plants already use renewable primary fuels such as biomass and biogas. With a strategy to slowly wean those CHP facilities using non-renewable fuels (such as natural gas) to renewable fuels (such as biogas) this scheme sets the stage for a future 100% renewable energy system for the whole of Denmark.

Technical Details for the NFC Integrated System
The NFC heating system employs 60 m2 of solar thermal panels in parallel with three renewable energy heat appliances. The heating appliances include a 45 kW wind powered electric boiler, a 28 kW (8 kW electric, 20 kW heat) plant oil fueled CHP, and a 50 kW wood pellet stoker boiler. The loop also employs a 10,000 liter thermal buffer tank. The system uses low pressure hot water in a district heating loop in order to provide space heating and domestic hot water to 3000 m2.
The four heat appliances and buffer system work together using sophisticated control logic in order to maximize renewable wind and solar energy. When solar heat and wind power are not available, wood pellets or plant oil can be deployed. From May until September thermal solar provides the majority of the thermal requirements for NFC. Excess heat not used during the day is stored in domestic hot water heating tanks and the 10,000 liter buffer tank ensuring that there is heat and hot water when the sun is not shinning. The wind boiler is controlled by two criteria. Its main function is to convert excess power produced by the two NFC wind turbines to thermal energy. These criteria are: (i) available power and, (ii) boiler temperature set point.
The power supplied to the boiler is calculated based on the power being produced by the wind turbines minus what is required by the center. Therefore if the center is currently using 10 kW and the turbines are producing 30 kW then 20 kW is available to the wind boiler if the temperature set point is not achieved. The control system ensures that no power is purchased from the grid for the purpose of heating. Only when the wind turbines produce over 55 kW is there any power shipped to the grid. This is important because when the power is used locally NFC avoids contributing to the major power spikes during periods of high wind.
The temperature set point of the wind boiler is set to 75 degrees Celsius in the winter and 50 C in the summer. The temperature needs to be set back in the summer in order to provide sufficient load to the solar system. If there is insufficient load in the summer a boiling condition may occur potentially causing damage to the panels. The plant oil CHP unit is set to startup only when there is both a power and heat requirement. This is important as the economics of burning plant oil for only power or only heat are negative. The final appliance in the system is the 50 kW pellet boiler which utilizes waste wood in the form of pellets. Above the pellet boiler is a 25 m3 storage bunker. The pellet boiler supplies heat to NFC and is programmed to turn on when the supply temperature falls below 58 C. This occurs on colder, less sunny days when there is less than approximately 8 m/s of wind. At this wind speed NFC's electricity requirements are met but there is insufficient renewable electricity to supply the required heat.

In Summary

As Denmark has led the world in the development of wind power it can now lead the world again by showcasing an ingenious approach to integrating an even higher percentage of renewable heat and power on a large scale. Integrating wind power with district heating systems is a powerful way to both manage the invariable nature of wind power as well as increase the overall contribution of renewably-generated heat and power. Most importantly, setting up this foundation and then shifting to renewable primary fuels is clever way to make the necessary transition to a 100% renewable energy system for the country.
As the NFC mandate is to highlight solutions in an energy constrained world this latest development shows that despite lack of government support, innovation in heating, transport and renewable power production continues. Maegaard believes that the only way to fully integrate renewables into society is to look at integrating multiple renewable energies in a holistic approach. No renewable energy solution can stand alone and therefore all renewable technologies and resources have to be mobilized. If we look to nature as our inspiration we can see that diversity builds strength and resilience and has worked like this for millions of years. Moving forward we need to build our lives and systems around similar principals.