Sustainable Heat & Power Europe GmbH

Although our services are not limited to solar thermal or Concentrating Solar Power (CSP) plants, the smart design of such systems have evolved to be our core competency. We have also develloped a couple of own concepts for small decentral co- and tri-generation plants, we call them MSTPP (Mini Solar Thermal Power Plant). For this reason our webpage contains some of our thoughs about the various options in CSP.

Mini Solar Thermal Power Plants

Introduction

The concept of mini solar thermal power plants (MSTPP’s) was first elaborated by SHP Europe in 2003. Basically, it’s a small thermal expansion machine driven by steam supplied from a solar collector field. The system can be combined on a project-by-project basis with a thermal storage system or a back-up boiler, as well as various processes such as an absorption chiller, an autoclave, a desalination unit, etc. The utilisation of the solar energy can either be done with a steam or ORC turbine, a steam engine or any other type of expansion machine. It is also possible to use a fluid as heat transfer medium. This is avoiding the complexity of a direct steam generating solar collector field.

In addition to the electricity, a MSTPP is able to provide valuable thermal energy that can be used as process steam (e.g. as the driver for cooling and water desalination applications). This thermal energy can be gained either by extraction from the power generating process, in the form of excess steam directly from the solar field, or by using the exhaust steam or heat of the expansion machine.

Since the system can be combined with other thermal energy sources or a thermal storage system, MSTPP's can be designed to be dispatchable up to base-load capability.

Configuration

A steam motor or steam turbine can be directly driven by the saturated or superheated steam produced in the solar collector. In case of an ORC unit, there are two separate cycles linked by a heat exchanger. The heat-transfer medium in the solar collector cycle can be steam, thermal oil or pressurised water.

This basic configuration of the MSTPP may be complemented by the addition of any of a number of systems:

  • Heat storage
  • Support or backup firing
  • Heat usage applications (process steam, thermal cooling, thermal desalination, heating)

The presence of a thermal storage or a boiler extends the system’s operating time, compared to a pure solar system, thereby increasing the availability and range of applications.

Thermal energy produced by the plant can be used for different applications at:

  • A high temperature level (of up and above 450 °C), directly from the solar field
  • At medium temperature (100 °C - 250 °C), using steam from extractions or the backpressure of a turbine,
  • At low temperature (of about 60 °C to 80 °C), utilising the waste energy of the electricity generating process.

Scheme of a MSTPP

Areas of Application

MSTPPs can be designed as grid connected, or island-grid cogeneration plants.

The main limiting factors for reasonable usage of MSTPP applications are the level or solar irradiation, the size of the available land area and the potential off-take for the available thermal energy.

Originally, such plants were thought to be economically feasible upwards of about 100 kWel. However, due to the massive cost reduction of photovoltaic systems in recent years, the focus of the MSTPP has shifted towards larger systems and applications in which the process steam is the primary goal and electricity production is a delightful bonus.

Due to the above-mentioned competition with PV-systems, MSTPP systems meant purely to generate electricity are only reasonable in larger applications (at minimum, 2 MWel to 5 MWel). However, if provision of thermal energy is the main purpose and economic drive of the plant, any size of expansion motor may be applied. The electricity generated by such as system is produced as a side product.

That said, it’s clear that MSTPPs fill the important gap between small-size photovoltaic systems and larger, centralised electricity generation facilities but with the advantage of a second product being present : thermal energy.

Product/System

By far, the largest component of an MSTPP is the solar collector field. Compared to its land requirement, the remaining power plant facilities require a proportionally small footprint. These other facilities are:

  • The unit that generates the electricity, including its auxiliaries and plant-control system
  • The cooling system, transformer and switchgear infrastructure for the grid connection
  • The thermal storage system, back-up boiler and waste-heat utilisation system (if applicable to the agreed design)
Spilling Motor

First, the mirrors of the solar collectors concentrate direct solar irradiation into an absorber, where a fluid is heated. (The type of heat transfer fluid and the reachable temperature is depending on the applied solar technology: Fresnel, or Parabolic Trough.) Preferably, the system uses water, which is then directly evaporated in the solar collectors. The steam produced by the system is then used to generate electricity.

Any steam from the collector field that exceeds the capacity of the plant may either be used directly for high-temperature applications (e.g. industrial processes in the food or chemical industry) or to transmit its thermal energy to a storage system that enables the operation of the MSTPP when solar irradiation alone is not sufficient to run the plant. The same benefit is achieved when a co-firing system (such as a biogas- or biomass- fired steam boiler) is applied.

Advantages

The main advantage of a MSTPP is that the system doesn’t just produce electricity. It also provides process steam (or heat), as well as chilled or desalinated water. The MSTPP produces these add-on benefits at the location where demand occurs.

Here are a few other possible applications for MSTPP:

  • Process Steam and Process Heat
    Process heat is used directly in industry to provide the thermal energy for endothermic processes (be it a required heat input for chemical reactions or heat for boiling, washing, drying, or sterilisation).
  • Cooling
    Another major application for process heat is its opposite: the provision of cooling. Here, the steam/heat is used to drive absorption chilling machines. The working principle is similar to that of the common compression chillers. The difference is that the thermal energy takes over the role of the electrically driven compressor, thereby saving valuable electricity. Absorption chillers can be single, dual-stage or even triple-stage. Lithium bromide commonly acts as absorption media for air-conditioning systems, and ammonia is used for dual- or triple-stage chillers in refrigeration applications.
  • Thermal Seawater Desalination
    A third application for process steam is the desalination of salty or brackish water. This is a very attractive option, as itís generally the regions with high solar irradiation that face supply problems with potable water for inhabitants and agriculture. One technology that uses thermal energy as the driver is the MED (Multi-effect Desalination) process. The process steam/heat used in the first effect of a MED evaporates brine, and the emerging clean vapour is then used in the next stage. (An increasing vacuum in the different effects drives this process.) One key advantage of the MED process is that it only requires heat input at relatively low temperature (70-80 °C), so itís able to use the waste heat of a combustion motor, the exhaust heat of a steam turbine, or the heat collected in ordinary flat-plate or vacuum-tube solar collectors.
  • Process steam provision for industrial consumers for food and chemical industry, mining, etc.
  • Air conditioning or refrigeration with absorption chillers (for shopping centres, hotel complexes, hospitals, food-industry installations, etc.)
  • Water desalination with evaporation systems (for new townships, major tourism projects planned in arid areas without sufficient drinking water supply, etc.)
  • Hot water (for laundries, kitchens, domestic hot water, room heating for hotels, hospitals, administration buildings, etc.)

Each MSTPP must be optimised for the individual application. Main design issues are the available solar irradiation and the temperature level and demand profile of the heat consumer.

Market/Support/Financial Aspects

MSTPP can be economically competitive with PV where the economic performance of the plant is mostly determined by the value of the produced thermal energy and/or waste heat.

Preferred applications for MSTPP's are:

  • Countries with high feed-in tariffs for solar thermal electricity or a bonus for co-generation
  • Remote areas without grid connection, in which the plant is in competition with diesel-fired generating systems and in which a need for thermal energy is present

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:

  • Solar Measurement Campaigns
  • Site assessment and project development services
  • Feasibility studies and analyses of the generation potential, technical concept and economic forecast for the MSTPP
  • Engineering services and project management for Mini Solar Thermal Power Plants