Comparing Solar Thermal and Solar PV Technologies for the Future

solar thermal energy
Combined cycle power plant in South Africa.

How will the technologies for the production of electricity from solar energy develop in the coming decades? Which technology is the most economical? What opportunities do the combination of multiple systems offer? A study conducted under the leadership of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) examined the future development of photovoltaic and solar thermal power plants up to 2030. The results were presented at the SolarPACES Conference from 11 to 14 October 2016 in Abu Dhabi.

Background

The objective of the study is to produce a systematic, technical, economic and environmental comparison of solar thermal and photovoltaic power plants. To this end, the THERMVOLT project analysed concepts such as how commercial solar power plants can produce low-cost and despatchable power, thus generating electricity reliably – even when the Sun is not shining.

Solar thermal power plants (Concentrated Solar Power; CSP) generate electric power by using mirrors to concentrate solar radiation and convert it into heat, which is either stored or used directly. It has various fields of application – from power generation to fuel production (hydrogen). Solar thermal power plants enable electricity production regardless of fluctuating solar radiation through the integration of thermal storage systems and/or a combination with fossil fuels or, in the future, biomass powered burners (hybridisation). As a result, solar power can be provided on demand and the base load capacity of the power plant can be achieved. Therefore, in terms of their value in the power system, these power plants are equivalent to large, conventional fossil fuel power plants.

Photovoltaic solar energy systems (PV) capture sunlight with solar cells and convert the solar radiation directly into electricity. This power can be used directly or stored in batteries. At present, the storage of PV power in batteries is less economical than storing solar thermal energy. In recent years, PV technology has significantly reduced its electricity production costs. However, it is one of the most unstable power feeders and therefore cannot ensure security of supply on its own. In addition, reserve fossil fuel supplies must be available to feed power to the electricity grid.

Recent studies have shown that with a greater proportion of renewable and variable electricity generation, the networks reach their limits. Therefore, when no storage is available, the proportion of renewable electricity must be limited.

Objectives and methodology

In the course of the study, the researchers simulated the costs of various PV and CSP-based power plant concepts as well as combinations of both technologies under the same conditions. The virtual power plants had to be capable of following various predefined load profiles and, at the same time, achieve the lowest possible greenhouse gas (carbon dioxide) emissions with the lowest electricity generation costs. The CSP power plants studied had thermal energy storage as well as a fossil fuel burner, which would only be used if needed. The PV-battery power plants had a battery storage unit and a fossil reserve system, for example a gas-fuelled power plant, which could be operated jointly.

The power plant had a capacity of 100 megawatts, and sun-rich regions such as Morocco and Saudi Arabia were studied as representative locations. The computer models included the years 2015, 2020 and 2030.

The calculations were carried out in detail for a whole year with a resolution of one hour, where the optimum size of the solar field and the storage was determined. To calculate the cost of electricity generation for the optimised systems, an economic model was created that took into account a number of effects (for example, wear) and various cost scenarios.

Efficient combination

The results show that, under present conditions, the combination of CSP and PV is more cost-effective in most scenarios than the use of only one of the two technologies. In this case, the PV part of the power plant feeds electricity directly into the grid during the day and the solar thermal component stores the solar energy in its thermal storage unit to convert it into electricity as needed, usually at night.

For high power requirements during night hours, CSP-based power plants are at an advantage because of their thermal storage. At the same time, a hybrid operation with fossil fuels or alternative energy sources can be integrated relatively easily at little extra cost.

While the photovoltaic systems had the highest electricity generation costs due to the more expensive battery storage costs in 2015, they could, under favourable conditions, approach or even undercut solar thermal power plants with thermal storage by 2030. This is however highly dependent on the relevant target market and the load specification.

The final study report will be submitted by the end of 2016.

Cooperation

The study received funding of approximately 500,000 euro from the German Federal Ministry of Economics and Technology (Bundesministerium für Wirtschaft und Energie; BMWi). On the scientific side, the DLR Institutes of Solar Research and Engineering Thermodynamics, as well as the Lappeenranta University of Technology (LUT) in Finland, were involved. Fichtner GmbH and the M+W Group GmbH GmbH also participated as industry partners.

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