A hybrid solution

A combination of approaches may offer the only way of incorporating more renewable energy sources into existing power grids.

Wind and solar power are frequently touted as being natural alternatives to fuel-based electricity generation, but both depend upon weather conditions. Reliable reserve power capability is, therefore, necessary to ensure efficiency of supply.

Rising fossil fuel prices, decreasing security of fuel supply, and the desire to limit greenhouse gas emissions are reasons the European Union (EU) has decided to stimulate an increase in Europe's use of renewable energy sources. The European Commission says that Europe has to derive 20 per cent of its energy from renewable sources by 2020. Part of this will be achieved in the transport sector, where 10 per cent of fuel demand will be covered by renewables such as ethanol or methanol. Most renewable energy will be used for electricity, and high hopes are put in wind power and photovoltaics. However, electricity generation based on wind power and solar radiation lacks the easy controllability of a fuel-based power generation, resulting in difficulties in meeting demand.

The use of electricity and human activities are closely connected. Electricity is a very versatile energy source used for artificial lighting, and to power production lines and public transport, for example. During the night, the demand falls to a minimum, but from 7am on weekdays many activities start up again, resulting in a sharp rise in demand that can be up to 180 per cent of the minimum demand. In hotter countries, air conditioning causes a peak in demand at noon. In colder countries, demand peaks during winter can be 50 per cent higher than in summer as electricity is used for domestic heating.

The demand fluctuations caused by individual users switching equipment on and off have a stochastic nature and do generally not exceed 2 per cent of the mean momentary demand. Network operators need to be able to control the output of the generators in a supply system to cope with these short-term fluctuations. These generators can then be used for frequency regulation, load following and as spinning or non-spinning reserve. Network operators predict the demand pattern for the next day and contract sufficient capacity to meet the sharp rises in power in the morning and the afternoon.

Steam-based power plants typically need a couple of hours for preheating before they can deliver electricity. Their ramping-up capacity is about 2-3 per cent of their nominal power per minute. Power stations can also fail to start up or trip at full load. That is why a contingency reserve is required, which can be spinning reserve (running and online) as well as standby reserve (non-spinning, but ready to come online). Non-spinning reserve power has to come online as soon as the bulk of the spinning reserve has been used. It will be clear that a generator owner has to be compensated financially for such so-called system services (or 'ancillary services' in the North-American literature). Reciprocating engines and aero-derivative gas turbines have a faster ramping-up rate than steam-based power systems and are often used for these type of services.

Generators based on hydro energy from water reservoirs can also react quickly to demand fluctuations. Countries such as Italy and Portugal have the landscape and climate to make extensive use of hydro power, even though the available energy varies from year to year and depends upon the time of year. Existing hydro systems make it relatively easy to apply pumped storage, so that excess electricity from other sources can be used to pump water into the reservoirs for later use. The energy efficiency of such pumped-storage systems lies between 75 and 80 per cent. In the EU, pumped storage capacity is about 4 per cent of the total installed generating capacity.

Production characteristics

Electricity production from wind energy and photovoltaics depends on weather conditions and is largely not controllable (non-dispatchable). If the wind increases in strength, there is a high risk that the wind turbines will suddenly have to cease operation while running at rated capacity, to avoid damage caused by overspeed. To cope with this, the network needs spinning capacity that can instantaneously take over the load from the wind turbines. Also, if the wind speed falls drastically over a wide area, much standby reserve has to be put online. In both cases, the network operator has to pay for these services, which can reduce the intrinsic economic value of the electricity from renewables down to almost nothing, therefore the real economic value of the electricity produced depends on the extent to which the network operator can use it to match demand.

Photovoltaic electricity peaks normally at noon, so it is of benefit only in situations where air conditioning requires much energy, such as in Spain in the summer. Solar power will not reduce the total installed generating capacity needed to meet the winter peak from 4pm to 9pm since solar irradiation is very low at that time.

Biomass-based power plants differ in this respect, however, since their fuel can be stored close to the power plant, making the output more dispatchable. Liquid biofuels are ideal for diesel engines that can act as spinning and back-up reserve for electricity from wind and solar. 

Wind power

Wind speed has a stochastic character. The average wind speed at offshore locations is normally higher than that at land-based sites, giving a higher capacity factor. The capacity factor of a windmill is the total electricity produced in a year divided by the electricity that would be produced if the generator was running 100 per cent of the time at full load.

A new offshore wind park of 110MW near the coast of the Netherlands (Shell/NUON) is expected to have a capacity factor of 30 per cent. The specific capital investment in that park is €2,100/kW. For a commercial fixed interest rate of 10 per cent, the specific capital costs will be 8 cents/kWh. Insurance costs as well as operation and maintenance costs will easily result in production costs of more than 11 cents/kWh. That is rather high for non-dispatchable electricity. Land-based wind installations are considerably cheaper than offshore installations (€1,200-1,500/kW), but their capacity factor is generally just slightly above 20 per cent.

Denmark has the highest relative share of wind-energy-based electricity. In 2004, 16.3 per cent of the total 40.5TWh electricity was from by wind power. However, the capacity factor for the total installed wind power was just 24 per cent. Assuming that, on average, 5 per cent of the windmills were not available for production because of maintenance, the available wind capacity was 0.95 x 3.1GW = 2.9GW. Total annual net electricity generation from all sources in Denmark is 38.4TWh, which means a time-averaged production of 4.4GW. It is easy to see that wind power in Denmark will exceed the nightly minimum weekday demand if the 2.9GW active wind capacity is online with a favourable wind. With a policy of unlimited grid feed in by wind power, the only solution in this situation is to export. That explains why Denmark has high electricity exports compared with four typical EU-25 countries.

The problem of having too much electricity production online is worsened in winter in Denmark since the demand for heating is often met by combined heat and power units. That means that especially during cold windy nights even more excess electricity is produced. If there are high winds, much fuel-based or hydro-based spinning reserve and back-up power has to be available in case the critical wind speed is exceeded. This is quite costly and results in spoiled fuel consumption.

On the other hand, wind speed levels are low for a significant part of the year, so power plants based on fuels have to take over. As a result, the utilisation factor for conventional thermal plants in Denmark is only 37 per cent, while the general optimum for the sector lies between 50 and 55 per cent. Denmark also actively uses electricity imports to cover peaks in demand. Denmark itself has no pumped hydro storage, but uses Norway's storage capacity to some extent.

Without wind power that can freely feed into the grid, nuclear and coal-based power plants normally cover the base load power generation, so that capacity factors of up to 85 per cent can be reached. However, with a high amount of wind power such as in Denmark, nuclear power is not attractive since it would have to be shut down during windy nights. The nuclear process is not suitable for this sort of operation, and it would also substantially increase the specific capital costs of electricity.

In summary, the stochastic character of wind energy makes it necessary to install additional dispatchable and flexible back-up power plants that will run with a low utilisation factor. Electricity in Denmark for domestic users cost 24 cents/kWh in 2004, twice as high as the EU-25 average. To produce 16.3 per cent of all the electricity used by wind, the wind capacity had to be 23 per cent of the total capacity, while the total installed generation capacity had a utilisation factor of 34.6 per cent.

Electricity from photovoltaics

The energy radiated to the Earth by the Sun exceeds the energy required for human activities many times. The challenge is in capturing the radiation. Current photovoltaic (PV) systems have an efficiency ranging from 10-15 per cent. In the Netherlands, with an average solar radiation of 115W/m2, the energy catching efficiency of straw is only 2 per cent and that of trees less than 0.3 per cent.

The main problems with PV, however, are the high capital investment of about €5,000/kW of rated power, and a capacity factor ranging from only 10 per cent in Denmark to 23 per cent in the Sahara. Under standard commercial financial conditions, this would result in specific capital costs of 58 cents/kWh in Denmark. Moreover, the electricity production of PV systems peaks at noon while it is relatively low in the wintertime. So the capacity is not available at the peak hours after 4pm and not at times of peak demand in winter. Since its production peaks at noon, proportionately even more peaking power capacity is needed.

Biomass solutions

In 2004, Denmark had a total of 474MW of electricity production capacity based on wood waste and 312MW based on municipal waste. Together these account for 5.9 per cent of Denmark's total generation capacity. The utilisation factor was 51 per cent, and it covered 8 per cent of the electricity need.

These steam-based systems have the advantage that the electricity produced is dispatchable and more controllable than that based on wind and solar radiation. However, although the fuel costs are low (and even negative in the case of waste), the specific investment is around €3,000/kW, resulting in specific capital costs of 7 cents/kWh, with specific maintenance costs of 2 cents/kWh.

The plants for the steam-based power lack the rapid load response needed for offering substantial spinning reserve and back-up reserve as network services. Nevertheless, they are an attractive way of using the energy available in solid biomass and waste. Small-scale wood-based generators in the power range 2-5MW are often used in forest-rich countries such as Finland and Sweden. In most cases, the most economic way to use biomass is co-firing in coal-fuelled power stations.

Biogas and liquid biofuels

Biogas primarily originates from sewage treatment plants and landfills, but digesters using farm and forest residues are also increasingly used.

Liquid biofuels are based on rapeseed, jatropha and palm oil or animal fats. Such fuels can easily be used with a high efficiency in reciprocating engines with a power capacity of up to 18MW. The specific capital investment in such decentralised installations (€/kW) is about the same as for large-scale generators. The small-scale character of these installations means that electricity can be produced close to the users so that the heat released can also be used (cogeneration), resulting in total fuel efficiency exceeding
85 per cent. The starting time of such installations is less than ten minutes so that their output can serve as back-up power in the electricity supply system.

Energy from the sun

Solar electricity may offer interesting options in the future to tap the energy influx from the Sun. At present, however, the specific investment cost as well as the amount of energy needed to produce the photovoltaic elements are far too high. Germany, for example, subsidises PV electricity with
50 cents/kWh, which is almost ten times as much as the costs of base-load electricity from existing power stations. This indirect way of stimulating the photovoltaic industry is counterproductive since it is a burden on the economy. It would be much better to directly subsidise extensive research into more cost-effective PV equipment.

The basic character of wind power means that the installed wind capacity cannot be considered as controllable, and sufficient alternative capacity is needed to at least cover the peaks in demand. Simple load shedding during conditions when the windmills cannot produce is generally not acceptable. Since wind capacity reduces the utilisation factor of non-wind capacity, it is necessary to have a back-up with a low specific investment. In addition, the non-wind generators need to have high ramping-up and ramping-down rates. That is required in times when wind power covers the base load (a maximum of 20-30 per cent of the time), since then the other generators have to take care of the rapid rise in demand associated with the intermediate load on weekday mornings. A rapid response is also required in case the wind power suddenly stops because of excessively high winds.

Fuels are certain to become scarcer during the planned life of at least 30 years for future generating equipment, so high fuel efficiency is very important. Power plants consisting of generators driven by reciprocating engines can have a simple-cycle electrical efficiency of up to 45 per cent, can ramp up to full power in two minutes (ramp rate 50 per cent per minute) and start from standstill to full load within ten minutes. Such units also have the advantage of a quite flat efficiency curve in the upper load range, which is attractive for offering spinning reserve. Moreover, the specific investments for gas-fuelled installations are only about €500/kW. Installations running on bioliquids can cost up to €700/kWh. An attractive option is the use of dual-fuel engines. Such engines can run on renewable liquid fuels if these are available, and can switch over to liquid or gaseous fossil fuel when they are not available.

The electrical efficiency of such installations can be further improved to roughly 50 per cent by adding a steam cycle or organic Rankine cycle. Until now, the additional investment of about €1,200/kW for such a topping cycle has been considered uneconomic. However, compared with wind power it is quite attractive, since here again the 'fuel' is free except in the case of high-temperature cogeneration. The capacity is also more controllable than that of wind power.

As mentioned before, using substantial wind capacity will reduce the utilisation factor of other installed generating capacity. That, in turn, will result in higher specific capital costs for the non-wind capacity. In an honest comparison, these extra costs should be attributed to that of electricity from wind. For gas-engine-driven installations, the specific capital costs of electricity production will increase from 1 cents/kWh to 1.5 cents/kWh if the utilisation factor decreases from 55 per cent to 35 per cent. The difference would be at least three to six times higher for coal-fuelled and nuclear power plants.

Power stations based on reciprocating engines have multiple units that run in parallel. That guarantees high reliability and availability. By using engines with a unit power capacity ranging between 5MW and 18MW, the size of such power stations can easily match the output of wind-power parks. The power stations can be built at suitable locations, preferably close to heat users for cogeneration. Generating units at different locations can even be combined into virtual power plants. The units based on reciprocating engines should be equipped with heat recovery as much as possible. Heat storage systems may be needed to make sure that sufficient heating capacity is available in case the cogeneration units cannot run during cold nights because much wind capacity is online.

In conclusion, liquid biofuel-fired and gas-fuelled power plants based on efficient and flexible engine-driven generators, with rapid starting and ramping-up capabilities, as well as a flat efficiency curve in the upper load range, create cost-effective and energy-efficient reserve power for wind stations.

Jacob Klimstra is a senior energy and engine expert at Wärtsilä Power Plants in The Netherlands

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