approximately 22 % of the total area and approximately 40% of the area where no restrictions occur has been utilized.

In addition to this we have assumed a further 15 TWh per year from areas with 6 7 m/s median wind. This is equal to approximately 2,150 4-MW plants covering an area of approximately 3,660 km2. This is approximately 1/7 of the area available (approximately 23,000 km2) (25) in this interval according to rough calculations (cf fig. 4.2).

The primary land use for this total of 3,700 4-MW plants is approximately 37 km2 (i.e. approximately 1 ha/plant).

In addition to these plants there are small generators of 8-40 kW, primarily for local needs (electricity or heat). They cannot be connected to the grid in order to use this as energy storage. Such plants ought to be of interest for homes outside urban areas, farms, etc.

Solar cells are assumed to be mounted on or near existing houses. They can be roofing of parking places etc. Central solar power stations have not been assumed. An area equal to approximately 1/10 of the urban area is assumed usable for solar cells (perhaps in combination with solar collectors for heat). The urban area in Sweden today is approximately 4,000 km2 and can be expected to increase. (As "urban area" is in Sweden all groups of houses with a total of not less than 200 inhabitants considered if the distance between the houses on the average is less than 200 m). The solar cells are primarily assumed to be mounted on suitable roofs and walls of houses. We have used a contribution from solar cells of 50 TWh per year.

Solar cells produce a direct current of low voltage. By connecting several cells in series the voltage can be increased. The power is converted to alternating current and can be transformed to a voltage suitable to the locality after which it is fed into the grid.

Figure 4.2:

Annual median winds at 100 m above ground level (25)

Energy from the sea can be produced either from waves (67) or from salt concentration gradients (70) (mixing of water with different salinity as occurs at river mouths). We have used a contribution of approximately 1 TWh per year from these systems.

Electricity in our system is produced by combined generation of heat and electricity, industrial counter-pressure and fuel cells apart from hydro-electricity, windpower, solar cells and aquatic

sources.

Electricity must be produced in the same instant it is used. This fundamental characteristic of electricity shapes the electricity system. The demand for electricity changes from one moment to the other. Production must thus be variable in the same way. The norma 1 method of achieving this is by allowing small changes in the load to lead to small changes in frequency. In this way e.g. a small increase in load is counter-acted because a small lowering of the frequency means that engines etc work somewhat slower and therefore need less energy. Since all units are influenced by a change in frequency even a small change can compensate for a significant change in load. Larger changes are compensated for by e.g. increasing the water flow in hydro-electric power plants or by starting up further power plants. As a final resort gas turbines are connected up. This can give a further increase in power during a limited time. In the electricity system of Solar Sweden, power is supplied from many different plants. They are connected up to a complete system. The availability of power will vary with wind and insolation. Variations can thus be both fast and large. This is counter-acted by having the plants spread out over a large part of the country. When a surplus is produced this is stored, in systems designed for this purpose, to be used at times when the production otherwise is insufficient.

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Combined generation of power and heat

A production of 13 TWh electricity per year has been assumed. This is equivalent to an installation of approximately 3,000 MW (el). This is 2/3 of the installation already planned for the 80's of combined generation with a reduction of capacity of approximately 65% due to an assumed lower heat load (better insulation etc) after the turn of the century. The remaining 1/3 of the district heating system has been assumed to be heated by solar power (cf section 4.4.2).

The design of the plants needs in the long run be based neither
of today's steam turbines or diesel engines, fired either by
biomass or powdered wood. Fuel cells may turn out to be an attractive
alternative. In that case the waste heat would be used for district
heating.

The plants for combined generation are assumed to have hot water accumulators. In this way they can contribute to evening out the load on the electricity grid.

Industrial counter-pressure

Fuel use in industry in some cases gives rise to large heat gradients, from which electricity can be produced. This is a method of making energy use more efficient. We have assumed 6 TWh of electric energy produced in industry. This is in accordance with SINDS prognosis (42).

Fuel cells

In principle a fuel cell is an inverted electrolytic cell, i.e.
the supplied gas (e.g. oxygen and hydrogen) is converted to
electricity (and water). The modules are therefore small. Larger
installations are achieved by coupling small modules in series. The
conversion efficiency to electricity is given as 40 % plus heat, low

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temperature 25 % and higher temperatures 25 % of the energy supplied (61). The power output can easily be varied. Fuel cells are thus useful for evening out load variations. We have assumed an electricity production of 24 TWh from fuel cells. Cf section 4.7.

Total production of electricity is shown in table 4.9.

The demand for electricity varies between day and night, weekdays and time of the year. Some of the renewable energy sources cannot be regulated with respect to the time when electricity is produced. Measures must be taken to balance demand against supply at any given time. How this regulation is to be achieved has not been studied in detail. There are problems both of storage over short periods (day/week) and of seasonal storage (from summer to winter). Seasonal storage of water power is already done in large dams.