INDEX:

ISCC (Integrated Solar Combined Cycles).

1. FRESNEL-BIOMASS HYBRID PLANT.
2. TOWER- COMBINED CYCLE HYBRID PLANT.
3. COSTS.

INTRODUCTION.

The hybrid plants are based on normal thermal centrals such as coal, gas, fuel, biomass and combined cycle, but the fact you get by hybridizing them is that part of the energy you needed to heat the steam comes from the Sun, thus saving fuel and emissions. Thanks to this combination we join the advantages of thermal fuel plants which allow generating energy continuously and solar thermal plants, zero fuel cost.

1.    ISCC (Integrated Solar Combined Cycles)

ISCC technology combines all the benefits of the solar energy with a combine cycle benefits. The solar resource partially replaces the use of fossil fuel and in this way also reduces the emissions. The solar field is based on cylinder-parabolic technology.

1.1 Conventional combined cycle.

        Conventional combined cycle plants consist of a gas turbine, a heat exchanger and a steam turbine. In the case of a hybrid solar plant ISCC, solar energy is used as auxiliary power that will increase the cycle efficiency and also it will decrease emissions.

1.2 Solar combined cycle.

        A hybrid solar combined cycle plant operation is similar to a conventional combined cycle. Fuel is normally burnt in the gas turbine combustor. The exhaust gases are directed to the heat exchanger. Heat is added from the solar field. As a result, steam generation capacity is increased and consequently an increased production of electricity in steam turbine.

 Figure 1. ISCC plant operation diagram.

1.3         Basic requirements for an ISCC installation.

        When the time comes for installing solar combined cycle hybrid plants, it is necessary to meet the following the requirements:

 
1) Topography: The area should be flat, preferably with a slope less than 1%.

 
2) Irradiation: normal direct isolation (NDI) should be as high as possible.

 
3) Water availability: water is needed to cool the energy block.

4) Electrical transmission: Power lines and transmission capacity are required to pass solar energy from the plant to the consumer.

Figure 2. Area occupied by an ISCC.

2.    FRESNEL-BIOMASS HYBRID PLANT

Figure 3. Fresnel Plant.

Currently one of the most hybridization systems under development is the Fresnel collector system with biomass boilers.

        The system is based as in the previous case. The heat obtained by the solar system serves to increase the steam temperature and in this way it saves on fuel. In this case the fuel that would used is biomass, thus Co2 emissions are decreased too.

Main advantages of the Fresnel solar thermal plants:

1) Robustness, while construction is low cost.
2) Its main components are steel, glass and water.
3) Efficient use of land so it requires less extension.
4) It is cooled by air so it requires a minimum use of water.
5) No toxic materials.
6) Easy protection from hail, dust and storms.

3.    TOWER-COMBINED CYCLE HYBRID PLANT

In this system, high temperature towers are used. Within them, the fluid gets heated to high temperatures thanks to a kind of solar furnace. The following steps are now common to any conventional thermal plant, by using the heat from solar energy to heat the water steam.

Figure 4. Solar furnace.

4.    COSTS

         SunsLab estimations, formed after the combination of CSP departments of two national laboratories: Sandia National Laboratories in Albuquerque, New Mexico, and the National Renewable Energy Laboratory in Golden (Colorado), currently solar thermal technologies offer solar electricity at the lowest cost in large-scale generation (10 MW and more).  Current technologies reach a cost between 2 and 3 dollars per watt. This means 9 to 12 cents per kilowatt hour of solar energy.

       However, there are several factors to consider. For example, in areas where the wind usually reaches its maximum efficiency during the night, it makes a good complement to rush solar hours. The challenge is to integrate both by looking for areas with good wind and solar resources without bringing the solar equipment (especially the mirrors) to excessive wind loads. The other issue is to devise a protocol for shorter times when there is sun and wind at the same time.

        Besides the solar field should not be distant. Otherwise, the heat transfer over long distances would be expensive. Regulatory institutions support is vital. Even plants with sun and enough space can not move forward until governments put a firm price on carbon emissions.