Category Control of Solar Energy Systems

Committed Production

In this case the plant managers have to take two types of decision. First, they have to decide the production for the day and then, they have to decide the energy to be delivered and stored at each time period.

The benefit for the time interval k can be computed as

B(k) = Pr(k)e1(k) + (Ps+(k) — !3stPs—(k))est(k)

— I Pr(k) — Pcontract (k)e2(k) (8.7)

where e1(k) is the price agreed for period k, est (k) is the price of the energy stored and e2(k) is the penalty paid for contract deviation.

The benefits obtained in the course of one day will be

24

£(24) = E Pr(i)e1(i) + Pstored (24)est(24) — — Pcontract (i)|e2(i) (8.8)

i = 1

There are two types of decision that have to be taken...

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Non-committed Production

If prices are constant, the best option would be to produce as much electrical energy as possible. Storage can be used in those situations where more solar energy can be collected than converted and delivered. This occurs because turbines and generators are designed for nominal conditions which are usually below maximum solar con­ditions. In some cases, due to faults or maintenance operations some parts of the energy conversion system of the plant are not fully operative and it is not possible to convert all the energy collected from the solar system and it has to be either lost or stored.

If prices vary with the time of day or if production is carried out with penalization errors, energy storage can be used to maximize profits...

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Power Conversion System

There are different methods for modeling the PCS, many of them based on manu­facturer specifications or historical data. A PCS is typically composed of a steam generator, preheater, steam turbine and refrigeration tower. The steam generator is fed by the hot HTF.

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Storage System

The main types of storage system have been analyzed in Chap. 1. In electrical grid energy sources, stored energy has to be used to balance the mismatch between the energy production from intermittent energy sources and the demand. Simplified models of oil and molten salt accumulators will be briefly studied in this chapter.

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Solar Field

The solar field is the solar energy collector mechanism and, therefore, one of the most important parts of a solar energy plant. Different models of solar collector fields have been developed in Chap. 4 that are useful for optimization purposes (mainly those based on first principles).

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Costs

Similarly to price models, energy cost predictions are fundamental for producers and consumers in order to establish their bidding and contract strategies as well as operating their installations. The cost model basically consists of transforming the model of electrical consumption to a cost model. Parasitic loads and personnel costs have to be added. For instance, operating cost for the SEGS trough plants have been estimated to be around 0.03 euros/kWh [416].

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Prediction Model for Electricity Prices

Electricity prices are set by bilateral contracts between electrical utilities and by the pool where producers and consumers submit bids consisting of a set of quantities of energy and prices which they want to sell or buy and the corresponding minimum or maximum prices. The contracted quantities and corresponding prices are determined by a central operator who takes into account the offering and selling bids. Usually the demand and offer curve crossover points determine the price of electricity. On some occasions, prices are fixed by the regulating agent using matching of buyers and sellers by a mechanism; the highest buying price (eb) is matched to the lowest selling price (es) at the mid point ((eb + es)/2)...

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Market

The prices of electrical energy on the intraday market depend on supply and de­mand. The offer curve used to be well known because it depended on only a few generating agents who, in the past, know their production capacities well in ad­vance (except for unforeseen failures). The renewable energy market, especially the solar energy market, is affected by policy mechanisms designed by governments to encourage the use of renewable energy sources, such as the feed in tariffs (FIT). In some countries, Spain for example, the owners or promoters of these installations can use two different energy selling modes: (i) To sell all the energy to an electrical utility at fixed prices. (ii) To sell the electricity on the intraday market.

The renewable energy installations can choose, for periods of n...

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