Solar Thermal Energy

The benefits of solar power are compelling: environmental protection, economic growth, job creation, diversity of fuel supply and rapid deployment, as well as the global potential for technology transfer and innovation. The underlying advantage of solar energy is that the fuel is free, abundant and inexhaustible: indeed, the total amount of energy from the sun received at the earth's surface is enough to provide for annual global energy consumption 10,000 times over.

As far as climate change is concerned, a solid international consensus now clearly states that 'business as usual' is not an option, and that the world must move swiftly towards a clean energy economy. Solar thermal power is a prime choice in developing an affordable, feasible, global energy source that is able to substitute for fossil fuels in the sunbelt regions of the world.

THE SOLAR THERMAL POWER 2020 REPORT

Greenpeace and the European Solar Thermal Power Industry Association (ESTIA) have together produced the report Solar Thermal Power 2020 in order to update understanding of the contribution that solar thermal power can make to the world's energy supply. The report is also a practical blueprint to show that solar thermal power can supply electricity to more than 100 million people living in the sunniest parts of the world within two decades.

Modern solar thermal power plants provide bulk power equivalent to the output from conventional power stations, and can be built in about a year. The aim of the Solar Thermal Power 2020 blueprint is to push further forward the boundaries of technological progress, and realize the subsequent benefits. There is no need to wait for a magical 'breakthrough': solar thermal power is ready for global implementation today.

Solar Thermal Power 2020 indicates that there is solid industrial and political commitment to the expansion of the solar thermal power plant industry. Furthermore, the report clearly shows that the current surge of activity in the solar thermal electricity sector is only a foretaste of the massive transformation and expansion it is capable over the coming decades. But although such reports provide a useful guide, it is people's actions which bring about actual transformation and expansion. Greenpeace and ESTIA are encouraging politicians and policymakers, global citizens, energy officials, companies, investors and other interested parties to support solar thermal power by taking specific steps. Concrete action can ensure that hundreds of millions of people receive their electricity from the sun, harnessing its full potential for a common good.

Following 2002's Earth Summit in South Africa, the Johannesburg Renewable Energy Coalition was formed, with more than 80 countries proclaiming that their goal is to 'substantially increase the global share of renewable energy sources', on the basis of 'clear and ambitious time-bound targets'. Political declarations mean little if not put into practice, and as a result, this report provides a blueprint for action that governments can implement, showing what is possible with just one type of renewable technology. Solar thermal power is a global-scale technology that has the capacity to satisfy the energy and development needs of the world, without destroying it.

POWER FROM THE SUN

Solar thermal power is a relatively new technology, but it has already shown enormous promise. With few environmental impacts and a massive available resource, it offers an opportunity to the sunniest countries of the world comparable to that presented by offshore wind farms in those European nations with the windiest shorelines.

Solar thermal power uses direct sunlight, and so as a result plants must be sited in regions with high direct solar radiation. Among the most promising areas of the world are the south-western US, Central and South America, Africa, the Middle East, the Mediterranean countries of Europe, and Iran, as well as the desert regions of India, Pakistan, the former Soviet Union, China and Australia.

In many regions of the world, one square kilometre of land is enough to generate as much as 100-200 GWh of electricity per year using solar thermal technology. This is equivalent to the annual production of a 50 MW conventional coal-fired or gas-fired power plant. Worldwide, the exploitation of less than 1% of the total solar thermal potential would be enough to stabilize the world's climate through massive CO 2 reductions.

TURNING SOLAR HEAT INTO ELECTRICITY

Producing electricity from the energy in the sun's rays is a relatively straightforward process. Direct solar radiation can be concentrated and collected by a range of concentrating solar power (or 'CSP') technologies, to provide medium- to high-temperature heat (the technologies are described in Volker Quaschning's article in the last issue of Renewable Energy World ).This heat is then used to operate a conventional power cycle, for example through a steam or gas turbine or a Stirling engine. Solar heat collected during the day can also be stored in liquid, solid or phase-changing media such as molten salts, ceramics or concrete, and it is anticipated that, in the future, it will be possible to use phase-changing salt mixtures. At night, heat can be extracted from the storage medium to run the combined-cycle steam turbine. Solar thermal power plants can be designed for solar-only generation, ideally to satisfy demand during daylight hours, but with future storage systems, their operation could be extended to baseload requirements.

Electricity from solar thermal power is also becoming cheaper to produce. Plants operating in the US state of California have already achieved impressive cost reductions, with today's generation costs ranging between US 10 cents and 13 cents/kWh. However, costs are expected to fall closer to 5 cents in the future. Advanced technologies, mass production, economies of scale and improved operation will together enable a reduction in the cost of solar electricity to a level competitive with fossil fuel power plants within the next 10-15 years.

TECHNOLOGY, COSTS AND BENEFITS

Four main elements are required to produce electricity from solar thermal power: a concentrator, a receiver, some form of heat transport or storage, and power conversion equipment much the same as that used in a fossil fuel-fired plant. The three most promising solar thermal technologies are the parabolic trough, the central receiver or solar tower, and the parabolic dish.

Parabolic trough

These systems use trough-shaped mirror reflectors to concentrate sunlight onto receiver tubes, through which a thermal transfer fluid is heated to roughly 400°C and then used to produce superheated steam. Parabolic trough systems represent the most mature solar thermal power technology, with 354 MWe of plants connected to the grid in Southern California since the 1980s, and more than 2 km2 of parabolic trough collectors. These plants supply an annual 800 million kWh - enough for more than 200,000 households - at a generation cost of about US 10-13 cents/kWh.

Further advances are now being made in the technology, with utility-scale projects planned in Greece, Spain, Egypt, Mexico, India, Morocco, Iran, Israel, Italy, the US and Algeria. Electricity from trough plants combined with gas-fired combined-cycle plants, so-called integrated solar combined cycle (ISCC) systems, is expected to cost 6 Eurocents/kWh in the short term and 5 Eurocents/kWh in the medium term.

Central receiver systems

Central receiver, or solar tower, systems use a circular array of large, individually tracking mirrors - heliostats - to concentrate sunlight onto a central receiver mounted on top of a tower. Heat is then transferred for power generation through a choice of transfer media. After an intermediate up-scaling of these plants to 30 MW capacity, solar tower developers now feel confident that grid-connected tower power plants can be increased to a capacity of 200 MWe, as solar-only units. Use of heat storage will increase their flexibility.

Although central receiver plants are considered to be further from commercialization than parabolic trough systems, solar towers have good longer-term prospects for high conversion efficiencies. Projects are in various stages of development, ranging from assessment to implementation, in Spain, South Africa and the US. In the future, central receiver plant projects will benefit from similar cost reductions to those expected from parabolic trough plants. It is anticipated that total electricity costs will drop to 5 Eurocents/kWh in the medium term.

Parabolic dish systems

These are comparatively small units that use a dish-shaped reflector to concentrate sunlight, heating gas or air to generate power in a small engine at the focal point of the reflector. The potential of these dish systems lies primarily in decentralized power supply, and remote, stand-alone power applications. Projects are currently planned in the US, Australia and Europe. In terms of electricity costs, an attainable, medium-term goal is a figure of less than 15 Eurocents/kWh.

DELIVERING ENERGY AND ENVIRONMENTAL BENEFITS

Current trends show that two broad pathways have opened up for large-scale delivery of electricity using solar thermal power. One is the ISCC-type hybrid operation of solar collection and heat transfer, combined with state-of-the-art, combined-cycle gas-fired power plant. The other is solar-only operation, with increasing use of a storage medium such as molten salt; this method enables solar energy collected during the day to be stored and then dispatched when demand requires.

A major benefit of solar thermal power is that it has little environmental impact, with none of the polluting emissions or safety concerns associated with conventional generation technologies. There is no pollution in the form of exhaust gases during operation, while decommissioning a system is unproblematic. Each square metre of surface in a solar field is enough to avoid the annual production of 250-400 kg of carbon dioxide. Solar power can therefore make a substantial contribution towards international commitments for reducing the greenhouse gases emissions that contribute to climate change.

THE GLOBAL SOLAR THERMAL MARKET

New opportunities are opening up for solar thermal power as a result of the global drive for clean energy solutions. Both national and international initiatives are supporting the technology, encouraging the commercialization of production. The Concentrating Solar Power Global Market Initiative was launched in October 2003 (see following feature by Fred Morse).A number of countries have introduced legislation that forces power suppliers to source a rising percentage of their supply from renewable fuels. Bulk power, high-voltage transmission lines from high-insolation sites, such as in northern Africa, could encourage European utilities to finance large solar plants, power from which would be utilized in Europe.

These and other factors have led to significant consideration of plant construction in the sunbelt regions of the world. In addition, interest rates have drastically fallen worldwide, increasing the viability of capital-intensive renewable energy projects. The 'race to be first' in this sector is demonstrated by the range of specific, large solar thermal projects currently planned. These include:

• Algeria - 140 MW ISCC plant with 35 MW solar capacity
• Australia - 35 MW compact linear Fresnel reflector (CLFR)-based array to preheat steam at a 2000 MW coal-fired plant
• Egypt - 127 MW ISCC plant with 29 MW solar capacity
• Greece - 50 MW solar capacity using steam cycle
• India - 140 MW ISCC plant with 35 MW solar capacity
• Israel - 100 MW solar hybrid operation
• Italy - 40 MW solar capacity using steam cycle
• Mexico - 300 MW ISCC plant with 29 MW solar capacity
• Morocco - 230 MW ISCC plant with 26 MW solar capacity
• Spain - two, 50 MW solar capacity using steam cycle and storage in solar-only mode
• US - 50 MW solar capacity using steam cycle; 1 MW parabolic trough using Organic Rankine Cycle (ORC) engine

THE FUTURE FOR SOLAR THERMAL POWER

A scenario prepared by Greenpeace International and the European Solar Thermal Power Industry Association (ESTIA) projects what could be achieved by the year 2020 given the right market conditions. This scenario is based on expected advances in solar thermal technology, coupled with the growing number of countries supporting projects in order to achieve both climate change and power supply objectives.

Over the period encompassed by the scenario, it is predicted that solar thermal technology will have emerged from a relatively marginal position in the hierarchy of renewable energy sources to achieve a more substantial status, alongside the current market leaders such as hydro and wind power. From a current level of just 354 MW, the total installed capacity of solar thermal power plants will have passed 5000 MW by 2015, according to the Greenpeace-ESTIA projections. By 2020,additional capacity would be rising at a level of almost 4500 MW each year. Other notable features of the scenario include the following:

• by 2020, the total installed capacity of solar thermal power around the world will have reached 21,540 MW
• solar thermal power will have achieved an annual output of more than 54,000,000 MWh (54 TWh) - equivalent to the consumption of over one third of Australia's electricity demand
• capital investment in solar thermal plant will rise from US$375 million in 2005 to almost $5.4 billion in 2020; the total invested over the scenario period would amount to $41.8 billion
• expansion in the solar thermal power industry will result in the creation of 200,000 jobs worldwide, even excluding those involved in production of the hardware
• the five most promising countries (in terms of governmental targets or potentials), according to the scenario, are Spain, the US, Mexico, Australia and South Africa, each with more than 1000 MW of solar thermal projects expected by 2020
• over the period up to 2020, a total of 154 million tonnes of CO 2 emissions into the atmosphere would be prevented, making an important contribution to international climate protection targets

A further projection is also made for the potential expansion of the solar thermal power market over the subsequent two decades, up to 2040.This shows that, by 2030, worldwide capacity will have reached 106,000 MW, and by 2040 a level of almost 630,000 MW will have been achieved. Increased availability of plant, because of the greater use of efficient storage technology, will also increase the amount of electricity generated from a given installed capacity. The result is that, by 2040, more than 5% of the world's electricity demand could be satisfied by solar thermal power. (see Tables 1a, 1b and Table 2).