Mikael Frejman & Adam Rajewski, Wärtsilä, Finland
Most cogeneration or combined heat and power (CHP) plants are employed in district heating or industrial applications.
Rinkàƒ¸bing combined heat and power plant in Denmark, owned by Rinkàƒ¸bing Fjernvarmevàƒ¦rk
District heating loads vary over the year as well as during each day, and seldom go hand-in-hand with the demand for electricity. Industrial processes can also experience variations in levels of demand. Wärtsilä reciprocating engine power plants equipped with heat accumulators are especially suitable for applications that require both operational flexibility and the efficient use of primary fuel.
From the feasibility viewpoint, using primary fuel efficiently à‚— maintaining high levels of total efficiency à‚— is essential when a power plant is being operated in accordance with the price of the electricity being sold. Within certain limits, disconnecting heat production from electrical power production can be achieved by installing a heat accumulator.
In most district heating applications, these are designed to have a storage capacity of seven hours.
Storing heat for later use
A correctly dimensioned cogeneration plant equipped with Wärtsilä engines and a heat accumulator offers two clear advantages. Not only will there be no need to run large existing district heating boilers at low loads during the summer months, there will also be no need to run the power plants’ engines at night or during other periods when the price of electricity is low.
The ability to shutdown existing boilers for periods of several months offers additional economic advantages. The flow diagram and heat balance for a modular district heating solution based on the Wärtsilä 20V34SG gas engine is shown in Figure 1. Employing units with an established track record, the CHP module located at the free end of the engine is an essential component from the viewpoint of heat recovery.
Figure 1: High efficiency gas engine power plant with heat storage
District heating return water first enters the lubrication oil heat-recovery heat exchanger, then the high-temperature charge-air cooler and the jacket-cooling heat exchanger, and finally passes through the exhaust-gas heat-recovery heat exchanger (the boiler). All the controls required to manage this process are available in the control cabinet installed on the module.
Seven hours of heat
The heat accumulator shown in Figure 1 is constructed in the same way as a normal fuel-oil storage tank but it has a thick layer of thermal insulation. The water in the top layer of the tank is maintained at 98 à‚°C. This helps to keep the top of the vessel filled with steam, reducing the amount of oxygen à‚— and associated corrosion à‚— to a minimum.
In most district-heating applications the optimal time for unloading the accumulator is the seven-hour period during the night when the price that electricity can be sold for is low. There are also applications in Denmark, for example, where accumulators are designed to be unloaded during weekends. The accumulator shown in Figure 1 has a volume of 1500 mà‚³ per Wärtsilä 20V34SG engine and is designed to have an unloading time of seven hours.
Increased interest in the utilization of solar power has resulted in a lot of development effort going into increasing the heat density of heat accumulators, but for applications involving short-term storage, water storage tanks remain a simple and effective choice.
Korea: the Cheong Soo Project
The Cheong Soo power plant, which will be handed over this month, will generate the electricity and district heat required for a new suburban area covering approximately 3 km2. The plant comprises two Wärtsilä 20v34SG engines equipped with heat recovery systems for district heating.
Korea’s tariff policy means that selling electrical power to the grid is not feasible. At night, the electrical load in suburban areas is so low that the amount of heat generated is severely reduced. Unfortunately, the amount of heat needed during this period in the 24-hour cycle is at a maximum. Installing a heat accumulator solves this problem, and allows the Cheong Soo plant à‚— essentially operating in island mode à‚— to run continuously at high levels of total efficiency.
- Electrical power at the alternator terminals:16.9 MW
- Electrical efficiency: 43.4 per cent
- Recovered heat: summer 14.2 MW / winter 3.6 MW
- Total efficiency in summer: 80 per cent
- Hot water accumulator tank: 3000 mà‚³
A solution for Poland
CHP plants equipped with heat storage would also appear to be a good solution for Central Europe, particularly for Poland where almost all power and heat generation is based on burning hard coal and lignite. Significant effort will be required to modernize the system for two main reasons.
Firstly because there are many old units which need to be replaced. And secondly because the national average of Poland’s specific carbon dioxide (CO2) emissions is very high à‚— almost twice that of Germany. Furthermore, many heating-only plants are equipped with old coal fired boilers, and these should be replaced by modern cogeneration units.
High-efficiency CHP plants running on natural gas à‚— such as those delivered by Wärtsilä à‚— would appear to be a perfect solution, but there is a significant problem. Electricity prices in Poland still correspond with the era of coal fired generation and are very low. The baseload electricity price is currently around PLN160/MWh (€36/MWh).
This means that gas fired power plants are not feasible because even a modern plant using Wärtsilä 34SG engines with a total efficiency exceeding 80 per cent cannot generate profit just by selling electricity and heat. The revenues from sales of heat and electricity would in fact be lower than the fuel cost, without including either service costs or the need to finance the investment.
Yellow certificates are valuable
Operators of high-efficiency gas fired CHP plants receive so-called ‘yellow certificates’ for electricity that is cogenerated with heat, and these certificates are valued at an additional PLN120/MWh. This additional element makes the plant quite profitable even at today’s low PLN/€ exchange rate.
There are however two conditions: firstly, use of the plant concerned must result in sufficient primary energy savings; and secondly, the average efficiency of the plant on an annual basis must exceed 75 per cent.
While Wärtsilä power plants meet the first condition easily, the second is much more tricky. A simple CHP system requires the plant to be operated exactly in accordance with the heat demand curve.
It is not possible to keep electricity generation steady and only vary the level of heat recovery à‚— operated in this way the plant’s efficiency would fall below the limit, resulting in lost yellow certificates.On the other hand, adjusting plant loading according to the demand for heat is not a good solution either. Not only is it awkward to implement, but predicting the volume of electricity that will be generated and selling it becomes more difficult.
Decoupling heat and electricity production
Heat storage capacity allows heat and electricity production à‚— or actually their delivery à‚— to be decoupled. An arrangement of this type means that the plant will always be running close to 100 per cent load, but only during certain hours. This in turn enables the operator to sell the energy being generated as peak load energy (on the next-day market).
Profits can be higher as the peak load price is often PLN250/MWh or more, and operators will still receive yellow certificates because their plants fulfill the annual efficiency requirement. With a temperature difference of 48 à‚°C between the stored hot water and the district heating return water, the heat energy density in this type of accumulator is 53 kWh/mà‚³.
While the maximum temperature of 98 à‚°C for open tanks is a limitation, the cost of constructing pressurized tanks increases rapidly as their volumes rise above 1000 mà‚³, making them an unattractive investment for this type of application. The roof of each storage tank is equipped with a water lock to accommodate both the over-pressure and vacuum conditions. Kept filled by passing the make-up water through it, this water lock is prevents air from entering the tank. To minimize any disturbance of water stratification within the tank, top and bottom diffusers are also installed.
Heat accumulators in practice
Power plants are dimensioned according to the expected heating load. In summer, for example, if the accumulator volume is optimized for seven-hour use the district heating network will have to consume this heat on a daily basis. The level of maximum heat production is chosen to be much higher than the expected daytime need so that the accumulator can be charged during daylight hours.
For a two-engine Wärtsilä 20V34SG gas plant, the total accumulator volume is 3000 mà‚³. The maximum charging and unloading speeds for such an accumulator tank are in the 18 MW range, which exceeds the maximum consumption of 12.3 MW by an adequate margin.
Running hours remain the same
In simple terms, adding an accumulator to a power plant means that the engines can be run with a continuous load at high levels of total efficiency during daytime, while the additional heat being generated is partly stored for use during the following night.
Overall running hours are no different to those of a plant without an accumulator because the hours when a plant is in operation are related to the price at which electrical power can be sold.
In winter, the presence of an accumulator can even-out some temperature peaks in the hot return water. This can be of importance when recovering energy through the cooling of engine lubrication oil.
In this two-engine case, the additional investment required for the accumulator will be paid off in just four years by the increased levels of power production in daytime (11 GWh/year) and the increased levels of heat production (10 GWh/year).