Sustainable Heat & Power Europe GmbH

Solar Thermal Add-Ons/Combined Systems


The principle of solar thermal add-ons combines a solar thermal collector field with an already-existing conventional power plant. The fuel of the power plant could also be provided by another renewable energy source (like biomass) or even nuclear, but the highest potential for these applications exists in installations using fossil fuels such as coal, natural gas or fuel oil.

The idea of the solar thermal add-on is to work as a “fuel saver.” The solar field produces hot water, saturated or superheated steam, which is then coupled or injected at an appropriate point into the water/steam cycle of the power plant. Thereby, the load for the conventional boiler is reduced and valuable fossil fuel is saved. The higher the temperature of the solar heated medium, the bigger is the benefit for the plant.

Recent years have borne several studies to support this concept for market introduction. Solar thermal add-ons minimise technical risks and optimise investment potential, as well as boasting excellent solar energy conversion efficiency.

The concept of solar thermal add-ones has stirred interest within various utilities and power plant operators. Power plant OEM companies even consider offering solar thermal add-ons as the future standard. In the years going forward, fossil power plant projects in sunny regions will be outfitted with these systems as a matter of course.

Areas of Application

Add On

Solar thermal add-ons are generally meant to be an enrichment to the huge fossil-fired power plant park that’s already installed (and scheduled to continue operation into the next several decades). There are many gas and oil fired power stations in the Near East - as well as many coal-fired power stations in southern Europe, Australia and the United States - that would be massively benefited by such an upgrade. This is especially true in the current moment, when prices for primary energies are rising, the avoidance of emissions is becoming more important and emission trading is a reality.

Due to current state of development of the CSP technologies, the easiest and most cost-effective application is the integration of a solar thermal add-on into the feed water pre-heating system of the fossil plant. There, it replaces a part of the extraction steam which would normally be taken from a bleed point of the steam turbine. But nowadays, the available solar collector technologies also permit the injection of saturated steam into the intermediate- or low-pressure steam drum and as well the generation of superheated steam at reheat line steam conditions.

All these options have one thing in common: the existing conventional boiler ensures that the steam turbine receives the required amount of steam at all times. Different to pure solar power plants, there’s no need for a complex thermal storage system (or an expensive back-up boiler).


The optimum CSP technology and solar collector field design depends on the injection or coupling pressure, as well as the temperature levels that are determined as ideal for the project. The high temperatures required by modern steam power plants currently can only be achieved by solar tower systems. The medium temperatures required for the injection of saturated steam (or for the solar feed-water pre-heating) aren’t as specific. (They may be reached by both Fresnel and parabolic-trough technologies.)

Due to the direct steam-generating capability at high pressure, Fresnel collectors offer the most extensive list of benefits as an add-on. These may even achieve the superheated steam pressure and temperature level necessary for injection into reheat lines, and can be directly connected to the piping system of the power plant without the need for additional heat exchangers.

Another advantage of Fresnel collectors for add-on applications is the fact, that they harvest more energy per year and covered surface area than parabolic trough installations. This is not only reducing the necessary interconnecting piping cost, but is also an important factor in the plant feasibility, as vacant land besides of power plants is often scarce and expensive.

The power generation industry has a low-risk approach, so it’s of utmost importance to solar add-on conceptualisation that the installation and operation of the device never endangers the existing plant and its operational features, availability and reliability.

While only a connection to the IP steam system of the power plants is reasonable with the current state of development, improvements to the technology are in process that will enable the provision of live steam at precisely the same conditions that fossil-fired boilers deliver. This further increases the possibilities of an already-vast number of potential plant upgrades.


The integration of a solar thermal add-on offers several advantages for the plant operator. Solar energy reduces the consumption of fossil fuel (and thereby the fuel cost and emissions) of the amended plant.

Solar energy may also be used in another way: if electricity production is kept constant, the plant saves fuel due to a reduced boiler load. But if the boiler load is kept constant, additional steam is available. In the case of solar feed water pre-heating the solar energy reduces the amount of steam that is usually bled from the steam turbine to heat the feed water. Therefore, more steam is available in the later stages of the turbine. That steam expands and is thus transformed into electricity. This method of operation enables the operator to increase the peak load capability of his plant at times when the demand in the grid is highest, as there is usually an excellent correlation between the electric load coming from the operation of AC (Air conditioning) devices in the grid and the availability of solar energy.

In both cases the specific CO2 emissions per kWh generated electricity is reduced. These advantages are clearly valued in economic parameters, and they don’t cause any disadvantages for the existing installation. A solar thermal add-on to the feed water pre-heating or into the cold reheat line requires only minor changes to the plant equipment. Operational flexibility, cycle efficiency, availability and plant reliability will all remain as before - if not improved.

Market/Support/Financial Aspects

Add On

Solar thermal add-ons offer several economic opportunities. These opportunities stem from:

  • The generation and sale of CO2 emission reduction certificates (e.g. CERís)
  • The reduction of fossil fuel cost
  • The increased production of high priced peak power electricity
  • The generation and sale of Green Electricity generation certificates (in markets with mandatory renewable energy targets)

Add-ons are the most cost-effective way to introduce solar energy into the bulk electricity market.

Our Service

As well as its many years of technical expertise, SHP Europe retains excellent resources in cooperation with our network of regional and international partners. We offer the following services and supplies:

  • Site assessment and project development services
  • Solar Measurement Campaigns
  • Feasibility studies and analyses of the generation potential, technical concept and economic forecast for the solar thermal add-on
  • Engineering services and project management for the integration of the solar thermal add-on
  • Carbon Management Services