Constructing a solar power plant - Essay

This is a report on the Andasol 1 Solar Power Plant located in Marquesado Del Zenete, Granada, Spain. The aim of this report is look at the technology involved; the construction of the plant; the operating capacity and environmental impacts on the area involved.

The information contained in this report was primarily gathered from the internet and also Moodle notes from class lectures. To begin this report, we discussed what areas were to be researched and drew a plan of what needed to be done, ensuring the work was evenly distributed between us.

Introduction:

The Andasol-1 Solar power plant is situated in the Guadix plateau, Marquesado Del Zenete, Granada, Spain. This is an area at an elevation of 1090 to 1100 metres above sea level and is free from shading. The site itself is situated next to a motorway and has no residential houses in the immediate vicinity, The nearest HV line is approximately 7km away from the site.

Solar energy is particularly suited to this area. This area is one of the sunniest in Spain, receiving approximately 300 days of sunshine a year. (See Fig 2-1).

The type of solar collector used here is the Parabolic Trough Collector along with Molten Salt Heat Storage. Parabolic Trough Collectors are the most common type used in solar thermal plants – one of the main reasons being they utilise a lens to focus the sunlight onto a cell, thereby reducing the amount of costly semi-conducting material PV material while collecting as much sunlight as possible. [i] 

Parabolic Trough Collectors work by using mirrored surfaces on a North-South Axis which then track the sun as it moves across the sky. The mirrors reflect the sunlight onto a tube which runs the length of the collector. This tube is known as an absorber tube and is filled with a fluid that absorbs the concentrated solar radiation and can reach temperatures of up to 400 Deg C. The Collector Field consists of hundreds of these collectors joined together in rows known as Loops and the collected heat is then passed to a steam generator or to the heat storage (molten salt). [ii] (see fig. 2-2)

Using molten salt, the heat collected by the collectors can be used to ensure efficient use of the turbines during the day or indeed be used to continue powering the turbines for up to 7 hours at night.

Molten salt is used to store heat for a number of reasons. It is capable of retaining thermal heat energy over a long period of time; it can be used at temperatures of 1000°F which is inline with efficient steam turbines and it a non-toxic and readily available material. [iii] 

At the Andasol-1 plant, the heat storage system consists of two 14m-high tanks that are 36m in diameter and a have a capacity of 28,500tons of molten salt. The molten salt itself is a mixture of 60% sodium nitrate and 40% potassium nitrate. Both of these substances are currently used as fertilizers and preservatives in food production. [iv] (see fig. 2-3)

Construction Phase:

The construction of Andasol 1 began in July 2006, (see fig. 3-4), the work includes stripping the land to lay the foundations to affix the steel supports, and the parabolic mirrors are carefully mounted on the support structures in a specially constructed assembly hall on site. The complete collector devices are then mounted in the solar field. Steel pylons are then used to anchor the metal support structures. (see fig. 3-5)

The 150m-long collector chains are set with hydraulic drives that have a precision close to a tenth of a millimetre. The 312 collector rows are fixed in a north-south axis and follow the sun’s path along a single axis therefore obtaining as much sun during one day. [v] 

The solar collectors for the plant total approx. 510,120 square metres (0.51 km2), which contains over 200,000 mirrors situated along 312 rows totalling up to an overall length of 24 km, also including 90 kilometres of absorption pipes. The total physical area of the plant including collector spacing, the storage tanks and turbine housing, etc. is 2 km2. [vi] 

The site is broken down into three different sections: solar field, storage and steam cycle/ power block. (see fig. 3-6)

Solar field

The Andasol power plant has a solar field that covers 510,120 square meters. The parabolic troughs are connected by pipes. The rows are set up on a north-south axis and follow the course of the sun from east to west. The parabolic mirrors are made of 4 millimetre thick, silver-coated, curved white glass. The silver coating has an additional protective coating. The specially designed absorption pipes absorb the solar radiation reflected to transfer the solar energy into a heat transfer medium located in the pipe, which in turn then transmits the heat into the steam circuit. Specialists assemble and check these collectors photogrammetrically to determine their precision in specially-constructed factory buildings before the collectors are brought to the field and anchored. [vii] 

Storage tank

The Andasol power plants have a thermal storage tank allowing the power plants to provide scheduled power. In order to fill the tanks while simultaneously operating the turbines, the solar field must be larger than that of a power plant without a tank; therefore annual operating hours of the plant at peak loads can be nearly doubled this way. The liquid salt thermal storage functions under atmospheric pressure and consist of two tanks per power plant, measuring 14m in height and 36m in diameter. Solar Two in Barstow, California served as the reference project for the thermal storage tanks in the Andasol power plants because it had the same salt mixture – even though the storage tank was smaller. [viii] 

Steam cycle/ Power block

Turbines, generators and plant periphery are conventional power plant components, similar to those used in fossil fuel power plants. At Andasol, the plant’s turbine has a capacity of 50 megawatts and is specifically designed to ensure reliable operations during the daily start-up and shut down of the plant. Siemens in Sweden constructed the turbines for Andasol 1. A substation has been built near the town of Huéneja about seven kilometres southeast of the power plant site for feeding the electricity into the grid. The power supply contract includes general technical standards and conditions for electricity production. [ix] 

The power plant finished construction in December 2008 at a cost in the region of €300 million. (see fig. 3-7)

Operating Characteristics:

The plant consists of 7,488 individual collector elements which each have a length of 12m and a width of 5.8m. Four of these elements are joined together in what are called loops and in total there are 156 loops of collector elements over an area of 1.3 X 1.5km. The total reflective surface are is 510.000m². The turbine capacity of the power plant is 49.9MW. The annual power generation for this plant, given the high rate of solar radiation in this area of Spain (2.1 – 2.2kWh/m²/a) is expected to be in the region of 179GWh.

That figure approximates to the energy needs of 50,000 homes of 200,000 people. [x] 

The breakdown of the site is as follows:

The parabolic troughs are set up in 312 collector rows which are connected by pipes. One row is made up of two collector units. The mirrors follow the course of the sun and reflect solar radiation onto the absorption pipe. The absorption pipes were conceived especially for use in parabolic trough power plants. Every unit has its own solar sensors and drives, in order to track the position of the sun. The units each have 12 collectors, which have 28 mirrors and 3 absorption pipes. The power plant requires 7,488 collectors. The storage tanks at the plants can operate even on overcast days or after sunset.

A small portion of the heat that is produced from the solar field is kept as liquid salt. The heat required for this is stored in a molten salt mixture, 40% potassium nitrate and 60% sodium nitrate. Both of these substances are currently used as fertilizers and preservatives in food production. During the pumping process between the two tanks, the molten salt mixture can also absorb extra heat at a temperature of approx. 290°C, where it is heated to a temperature of 390°C. A full storage tank can be used to operate the turbine for about 7.5 hours.

The thermal storage tank allows the power plants to supply electricity regularly into Spain’s high-voltage electricity grid. The Spanish energy supplier Endesa purchases electricity from the power plants in accordance with Spanish energy law, which states that the power plants are permitted to feed in a maximum of 50 MW into the grid. The power supply contract includes the implementation of programming and the after-sales service for electricity production

Below is a further breakdown of some figures associated with the individual sections of the plant. (See Fig. 4-8)

The power plant was officially connected to the grid in December 2008 and became operational in March 2009. The total cost is somewhere in the region of € 300 million. A grant was also given from the European Union, which amounted to € 5 million; this was giving as a funding aid for scientific research. [xi] 

Environmental Impact:

The solar power plant will be able to cut carbon emissions in the region of 86,000t per year. The plant uses an unlimited energy source; no additional resources are needed for operation. It is CO2 and emission-free, therefore there is no air pollution (see Fig. 5-9). [xii] 

Environment and sustainable development needed to concentrate on some key areas: Sustainable management and water quality, global climate change, biodiversity, marine ecosystems, and the regions cultural heritage. Socio-economic aspects of environmental change with sustainable development in mind i.e. impact on, the economy and employment. [xiii] 

Another aspect is that when the plant is decommissioned, the salts from the storage tanks can be crystallized and removed in their raw state to be used thereafter in other applications e.g. in agriculture.

One of the main reasons for erecting the plant in this location was that it is close to existing road and railway infrastructure, and that it had the disposal of a flat land floor without environmental protections, it was considered wasteland. Another reason was the advantages offered by solar-thermal power plants, the operator of Spain’s national grid, Red Eléctrica de España (REE), classified Andasol power plants as “predictable” sources of electricity. This helped eased the permit process for granting access to the high-voltage grid, but it also made it possible to increase the percentage of renewable energy in the state’s energy mix because of the stabilizing effects of solar-thermal power plants. [xiv] 

Conclusions:

Spain is highly dependent on imported energy. Almost all petroleum and natural gas, as well as 70% of its coal, have to be imported. However, it has more than enough of one of the most environmentally friendly and cheapest sources of energy in the world – the sun. The Spanish government’s support plan “Plan de Fomento de Energías Renovables (PER)” envisions the expansion of solar thermal power plant capacity to 500 MW by 2010. Spain’s Ministry for Energy and Industry wants to increase the annual volume of electricity produced in solar-thermal power plants to about 4,000 GWh, which is the equivalent of the capacity of about 25 Andasol power plants. [xv] 

Andasol 2 followed in 2009 and Andasol 3 will follow in 2011. The Andasol location will result in a total of 150 megawatts of solar thermal power station capacity being connected to the grid – electricity generated in an environmentally-friendly and sustainable manner.