Add-on/
combined systems

Mini Solar Thermal Power Plants (MSTPP)

Introduction

The MSTPP directly competes with large scale photovoltaic systems above 100-200 kWp, but different to PV plants the MSTPP offers additional valuable thermal energy as a by-product. And due to the inherent possibility of cheap and simple energy storage and combination with other thermal energy forms also dispatchability up to base load capability is possible.
These are two distinct advantages compared to large scale PV systems.

Basic configuration

The Mini Solar Thermal Power Plant (MSTPP) in its basic configuration is a combination of a concentrating solar collector and a steam motor with its infrastructure to produce electricity. The solar collector produces saturated steam with a temperature level of about 290°C that is directly used in a steam motor.

Advanced configurations

In advanced configurations MSTPPs can be complemented by a heat storage and / or various heat usage applications to the basic system.
The heat storage helps to use solar power that exceeds the motor capacity and to extend the operating time of the system beyond the time of minimal necessary solar irradiation.
Heat usage applications can be on

• a high level temperature of 290°C using steam that comes directly from the solar collector
• a mid level temperature up to 95°C, using the waste heat that leaves the outlet of the steam motor.
• A low level temperature of about 60°C leaving the motor cooling system

Beneath you can find more detailed examples for heat usage applications.

Areas of application 

MSTPPs can be applied for stand alone or combined electricity generation and additional heat supply at different temperature and pressure levels. Thus, they offer a smart solution for a wide range of potential application cases.

The limits of reasonable applications for MSTPPs are mainly determined by the minimal and maximal sizes of available steam motors, the available surface area and the possible heat applications for the produced steam.

The minimal available steam motor size applicable for a MSTPP is designed for a minimal electrical capacity of 100kWel. Due to the various advantages of MSTPP systems in comparison to photovoltaic systems that are explained more detailed beneath we would always recommend a MSTPP system in situations where the waste heat can be utilized.

The upper limit for MSTPP is set by the availability and cost effectiveness of steam turbines appropriate to the characteristics of solar steam generation. The usage of steam motors offers a superior solution up to around 5 MWel in comparison to affordable saturated steam turbines.

Resuming one can say that MSTPP fill an important gap between small decentral photovoltaic systems and big central electricity generation facilities applying huge areas for solar collectors and turbines in add-on, combined or solar thermal stand alone systems.

Product/System 

The MSTPP to its largest portion consists of a Fresnel Reflector collector field. The remaining power plant facilities will cover about 200 m². These other facilities are a steam motor unit including auxiliary systems and plant control system, the cooling system, the transformer and switchgear infrastructure for the grid connection, and if applied, the thermal storage system.

The Fresnel reflectors concentrate the direct solar radiation to an absorber in which water is heated and evaporated at about 290°C. The steam in turn is converted into mechanical energy through a spilling steam motor and then into electricity using a conventional generator. The vapour/steam from the collector field that exceeds the motor capacity can either be used directly for high temperature applications as for example industrial processes in the food or chemical industry or transmit its heat to a storage system that assures that electricity can even be produced when solar radiation is not sufficient to produce electricity. The same benefit can also be derived by applying a co-firing system such as a biogas or biomass fired steam boiler.

The exhaust steam from the steam motor can be applied as driving fluid for processes such as absorption cooling or water desalination. Furthermore, the low temperature heat from the motor cooling system can be used for purposes such as room or water heating.

Advantages 

The big advantage of MSTPP is that it produces more than just electricity. MSTPP are an ideal instrument to provide electricity, heat, cooling and other things at the same time and at the location of demand.

Possible heat applications are for example:

• Water Desalination with evaporation systems for new townships, major touristic projects in arid areas without sufficient drinking water supply, etc.
• Air Conditioning with single stage absorption coolers for shopping centers, hotel complexes, hospitals, etc.
• Refrigeration with two stage absorption coolers for hospitals, food industry, etc.
• Process steam provision for food or chemical industry
• Hot water for laundries, kitchens, domestic hot water and room heating for hotels, hospitals, administration buildings, etc.

Since the location always determines the quantity of heat and the temperature level that is needed, each MSTPP must be optimized to the individual application.

In comparison with big photovoltaic systems of more than 100kWp MSTPPs have various advantages and possibilities in locations with a high amount of direct solar irradiation:

• The electricity that is produced is cheaper in comparison with photovoltaic systems
• The produced heat offers various possibilities which have the potential to further reduce the costs of the produced energy
• The combination with a thermal storage raises furthermore the potential to improve the economic performance and the flexibility of the system
• The possibility of the combination with other installations e.g. biomass co- or backup firing offers greater operational flexibility
• The possibility of producing main components of the system near the location supports the local economy
• Local energy production minimizes the transport losses

All these points lead to the conclusion that the MSTPP should be preferred in comparison to photovoltaic plants where systems of sizes bigger than 100kWel are required and wherever there is a coincidence of additional cooling, heating or desalination demand.

Market/Support/Financial Aspects  

Due to its size Mini Solar thermal power plants have high specific generation costs. These costs are already lower compared to PV systems. Consequently, if the guaranteed feed-in tariffs for PV such as in Portugal or Spain would also be applied to MSTPP’s, their economics would be superior to PV. However, in the absence of such beneficial feed in tariff for small scale solar thermal electricity economic viability is dependent on a second revenue stream for the produced waste heat or the products derived from it. Rising fuel prices as we experience them now will make the option of waste heat usage even more attractive and improve the economic efficiency even more.

In all countries the situation has to be analyzed case by case, but a further beneficial economic effect may result from the below variety of options as

• Reduction of fossil fuel cost for existing or planned applications such as desalination or absorption cooling plants
• Possibility to sell green power from a small distributed plant as high profile electricity product
• Increased sales price for electricity due to the distributed nature of the plant feeding in into the distribution voltage network with reduced transmission losses
• Generation and sale of Green electricity generation certificates in markets with mandatory renewable energy targets
• Generation of CO₂ emission reduction certificates (e.g. CER’s) or possibility to sell unused CO₂ emission certificates (e.g. EU-Allowances)

Our Service 

Assisted by a network of regional and international cooperation partners, Solar Heat & Power Europe GmbH has the technical know how and resources to offer the following services and supplies:

• Site assessment and project development services
• Feasibility studies and analysis of the generation potential, the needed technical concept and the economic outcome of MSTPP
• Design and project management for Mini Solar Thermal Power Plants

Example 

The following example was presented during the World Renewable Energy Congress in Aberdeen in May 2005. It is a rough estimate for a simple plant installed in Southern Spain.

Since every location will be different, we highly recommend you to approach us for an individual investigation of your case.