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INTRODUCTION AND SCOPE
Solar thermal systems are one of the most promising modalities of Renewable Energy Sources in regions endowed with high values of Direct Normal Irradiation (DNI). Concentrating the solar radiation leads to high temperatures on the receiver, and therefore to the potential of high thermodynamic efficiency. Additionally, thermal energy readily lends itself to storage, thus making Concentrated Solar Power (CSP) a lead candidate for providing dispatchable energy from a renewable source.
Cyprus is a small island state, with an isolated electricity grid, which relies almost exclusively (91.9% in 2013) on imported heavy fuel oil for its electricity production. Also, Cyprus’ semi-arid climate necessitates seawater desalination to provide adequate fresh-water resources to its population. Climate projections indicate both an increase in temperatures (~ 3 °C) as well as a decrease in precipitation (~ 25%) by the end of the century, further increasing the strain on the electricity and water resources of the island. The case of Cyprus is not unique; several countries in the Middle East and indeed around the world face similar circumstances.
COGENERATION PILOT PLANT
The PROTEAS Facility (Platform for Research Observation and Technological Applications of Solar Energy) is a major infrastructure for research, development and testing of technologies relating to concentrated solar power (CSP) and solar seawater desalination. It is located at the south coast of Cyprus near the sea and it provides a test facility specializing in the development of CSP systems suitable for island and coastal environments with particular emphasis on small units (<25 MWth) endowed with substantial storage, suitable for use in isolation or distributed in small power grids.
The first major experiment taking place at the PROTEAS facility concerns the development of a pilot/experimental facility for the co-generation of electricity and desalinated seawater from CSP.
Specifically, the experimental plant consists of a heliostat central-receiver system for solar harvesting, thermal energy storage in molten salts (“solar salt”, 60-40% b.w. of NaNO3-KNO3), followed by a Rankine cycle for electricity production and a Multiple-Effect Distillation (MED) unit for seawater desalination. These technologies were selected after an extensive technical and economic study lead by The Cyprus Institute, which concluded that they are the most suitable for the particular conditions of grid-isolated island communities in general and Cyprus in particular. The experimental plant is meant to verify the concept, modeling and component behavior of a prototype design of a 4 MWe commercially viable plant.
The heliostat-central receiver technology was selected as the most appropriate for the conditions of Cyprus. A heliostat field was designed and built in collaboration with the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia employing CSIRO’s proprietary focusing heliostat design. The heliostats are installed on a hilly terrain, typical for the coast of Cyprus, with the field layout and terrain elevation contours. The field layout was optimized to maximize annual energy yield and minimize shading. Currently the field consists of 50 heliostats, each with a reflective area of 5 m2 and constructed out of a single mirror facet. Each mirror has a reflectivity of 93% and was pre-stressed to form a paraboloid of revolution. The heliostats employ spinning-elevation tracking, while a beam characterization system sequentially calibrates each heliostat throughout the day to minimize tracking error. The central receiver is placed on a 14 m tower. The receiver is a cavity type receiver with a circular aperture of 0.8 m in diameter.
Integrated Storage and Receiver (ISTORE) Unit
A novel device integrating the receiver and storage functions, named Integrated Storage and Receiver (ISTORE) was designed and constructed for the purpose of this experiment. As the name suggests, the aim and innovation of the design is to allow the merging of the thermal energy storage and receiver functions of a CSP point focusing system in one unit, therefore, reducing complexity, operational and capital costs. Solar radiation is directed from the heliostat field onto the internal surface of the cavity of the ISTORE. The principal cavity is of cylindrical shape expanding in four secondary cavities on the backside. The secondary cavities help to improve the capturing efficiency of the device and provide a larger surface area for heat transfer between the absorber and the heat transfer/storage fluid. The external surface of the cavity is in contact with the molten salt in the storage tank that surrounds it. The receiver is made of AISI SS321H and uses solar salt as the heat transfer fluid and storage medium.
In this first realization of ISTORE the energy storage capacity of the device is small and molten salt is recirculated to a separate storage tank. It is admitted to the device through a set of nozzles located at the backside and bottom of the cavity external shell. In order to improve further the thermal efficiency, the design allows for suction of hot air from the cavity, which is principally used to preheat seawater for the MED desalination unit. Other features of the ISTORE include a set of electric heaters which are used for preheating during the start-up phase as well as ports for various sensors (e.g. temperature and fluid level sensing) and a man-hole for inspection and maintenance work
Thermal Energy Storage
One of the aims of the PROTEAS demonstration facility is to show the feasibility of co-generation of electricity and desalinated seawater on a continuous (24/7) basis; therefore thermal energy storage is necessary to satisfy this requirement. A single-tank thermal energy storage (TES) systems using molten salt as the heat storage medium was designed and constructed in collaboration with the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA-Casaccia).
Molten solar salt was chosen as both the heat transfer fluid and thermal storage medium, after considering the operational temperature range of the mixture, its volumetric heat capacity, as well as economic considerations. The tank has a height of 2.8 m and volume of 8 m3, and is designed to operate at temperatures up to 600 °C in a non-pressurized environment, resulting in a total thermal storage capacity up to 0.6 MWh. The tank is constructed out of AISI SS321H to ensure compatibility with the molten salts and minimize corrosion. The tank is insulated with a total of 0.4 m of ceramic wool and rock wool insulation, to minimize thermal energy losses to the environment. Additionally, the tank support is interfaced to the main vessel through calcium silicate bricks, again to minimize the thermal losses through conduction. Five electrical heaters, with a total capacity of 45 kW, are installed as a backup to maintain the salt in a molten state at all times.
Steam and Electricity Production
Steam and electricity production units were added to the experiment to demonstrate a viable path towards the cogeneration of electricity and desalinated seawater. The currently implemented electricity production in this first phase of the experiment is intented for demonstration-pedagogical purpose; a more powerful unit will be installed in the second phase of the experiment. A forced circulation steam loop was designed, circulating water in a heat exchanger immersed in the molten salt to create saturated steam. This steam loop, which is in essence a Rankine cycle design combined with water desalination. A 10kWth/1.5kWe steam engine was procured and a custom 4-effect MED unit was developed and constructed for the co-generation of electricity and desalinated seawater, respectively.
The exhaust steam from the turbine is used as thermal input to the desalination, while the remaining energy from the steam is used to preheat the seawater for the desalination process. In regard to thermal energy for driving the Rankine cycle, a heat exchanger consisting of a pair of coils was designed to preheat water from room temperature to 200 °C (saturated liquid state) and then steam at a temperature between 270 °C to 500 °C, depending on the solar salt temperature.
A custom designed 4-effect distillation unit (MED) was constructed to operate either in series or in parallel with the steam engine. The MED unit utilizes a low-temperature enthalpy source (as low as 70 °C) to evaporate a quantity of seawater, yielding distillate vapor and a pool of brine. The process occurs in a partially evacuated chamber, to reduce the boiling temperature of seawater. The produced vapor is ported to the next effect, where the latent heat released by its condensation is used to evaporate a new amount of seawater. The present MED realization, uses the forward feed configuration, where the brine pool from one effect is used as the seawater for the next effect. The advantage of this method is that preheating the seawater from ambient to effect temperature is no longer necessary, as the brine pool is already at a higher temperature, however the brine also has a higher salinity and therefore more energy is required for its evaporation. Additionally, this unit employs plate heat exchangers instead of the traditional shell-and-tube type heat exchangers, to obtain larger specific heat transfer area in a more compact design.
The unit is designed to operate with a 10 kW thermal input and to produce up to 2 m3 of distillate product per day. The unit has been tested in a controlled environment (e.g. not coupled to a solar thermal process) and has achieved a gain output ratio of 2.7 (~240 kWth/m3) with four effects.
Integration and Controls
The Control System for the PROTEAS facility has been designed taking into consideration all architectural needs for a modern and robust system: A decentralized architecture based on Server/Client techniques with well-defined and easily separable control layers using network communication. The architecture is open, easily expandable and reconfigurable, as it is appropriate for an experimental setup. It monitors all subsystems of the experiment provide control and safety alarms as needed. The slow control nature that characterizes all crucial control processes of the facility permits periodic handling of the I/O signals in the order of several ms, preventing in this way the need of complicated real time loops. The system is also flexible and adaptable, as it concerns an experimental system which is expected to be changing configuration often with the introduction of new elements. The conceptual design of the Control System comprises several local control stations directly connected to various hardware units. The Control System has been realized using the commercially available MAQ20 industrial system by DATAFORTH. For network communication purposes special modules based on the ModBus protocol over Ethernet have been developed. The User Interface is being designed on the LabVIEW environment
The new field facility for solar research at Pentakomo Cyprus, has become recently operational and it offers a unique environment for testing in realistic coastal - island conditions solar technologies, in particular for electricity production and solar desalination. The PROTEAS facility, including utilization of the solar field of heliostats, its tower, molten salt other associated equipment are open to the local and international community, based on the merit of the proposed experimental program.
The first major experiment (CSP-DSW) to be conducted at the facility is the testing of the Cogeneration of Electricity and Desalinated Sea Water using Concentrated Solar Power. This experiment, if successful is intended to lead to the construction of a pilot facility (in the range of 2 to 8 MW), which should be economically viable. All components are installed and the entire experimental demo facility is undergoing commissioning. Definitive measurements, which are expected to be completed during the next twelve months, are expected to validate the concept and calibrate the extensive modeling that has been implemented. This will allow the design of the next phase of the project with increased confidence.