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Optimization study
This publication has been produced with the �inancial assistanceof the European Union under the ENPI CBC Mediterranean Sea Basin Programme
Disclaimer
This document has been produced with the �inancial assistance of the European Union under the ENPI CBC Mediterranean Sea Basin Programme. The contents of this document are under the sole responsibility of STS-Med Consortium and can under no circumstances be regarded as re�lecting the position of the European Union or of the Programme’s management structures.The total budget of STS-Med project is 4.953.513 Euro and it is �inanced for an amount of 4.458.162 Euro by the European Union through the ENPI CBC Mediterranean Sea Basin Programme (www.enpicbcmed.eu).
The Programme
The 2007-2013 ENPI CBC Mediterranean Sea Basin Programme is a multilateral Cross-Border Cooperation initiative funded by the European Neighbourhood and Partnership Instrument (ENPI). The Programme objective is to promote the sustainable and harmonious cooperation process at the Mediterranean Basin level by dealing with the common challenges and enhancing its endogenous potential. It �inances cooperation projects as a contribution to the economic, social, environmental and cultural development of the Mediterranean region. The following 14 countries participate in the Programme: Cyprus, Egypt, France, Greece, Israel, Italy, Jordan, Lebanon, Malta, Palestinian Authority, Portugal, Spain, Syria, Tunisia. The Joint Managing Authority (JMA) is the Autonomous Region of Sardinia (Italy). Of�icial Programme languages are Arabic, English, French and Greek.Launched in May 2011, the strategic call focused six topics chosen by the Joint Monitoring Committee based on their potential for the development of cooperation in the Mediterranean area. These are: agro-food industry, sustainable tourism, integrated coastal zone management, water management, waste treatment and recycling, solar energy. Out of 300 proposals presented, 19 projects were approved for funding. Total value of these operations is € 82.5 million (€ 74.1 million ENPI contribution).
The European Union
The European Union is made up of 28 Member States who have decided to gradually link together their know-how, resources and destinies. Together, during a period of enlargement of 50 years, they have built a zone of stability, democracy and sustainable development whilst maintaining cultural diversity, tolerance and individual freedoms. The European Union is committed to sharing its achievements and its values with countries and peoples beyond its borders.
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STS-Med Small scale thermal solar district units for Mediterranean communities Ref. I-A/2.3/174
Optimization study issued by ENEA - Italian National Agency for New technologies, Energy and Sustainable Economic Development December 2015
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Contact person for this report
Dr. Alberto Giaconia, Ph.D.
ENEA – Casaccia Research Center Energy Technologies Department (DTE) Solar Thermal and Thermodynamic Division (STT)
Email: [email protected]
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ACRONYMS and NOMENCLATURE
CPV: Concentrating Photo-Voltaic
CS: Concentrating Solar
CSP: Concentrating Solar Power
DNI: Direct Normal (solar) Irradiation
EE: Electrical Energy
HTF: Heat Transfer Fluid
MED: Multi-Effect Desalination
ORC: Organic Rankine Cycle
PCM: Phase Change Material
PV: Photo-Voltaic
RE: Renewable Energy
R&D: Research and Development
RD&D: Research, Development and Demonstration
TES: Thermal Energy (heat) Storage
TRL: Technology Readiness Level
WP: Work Package
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Table of contents
1. INTRODUCTION ....................................................................................................................................... 5
2. System integration and optimization ......................................................................................................... 5
2.1 Optimization of the solar field .......................................................................................................... 10
2.2 Optimization of the CS plant with thermal energy storage .............................................................. 11
2.3 Energy dispatchability and techno-economic issues ....................................................................... 14
3. SUMMARY AND CONCLUSIONS .......................................................................................................... 17
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1. INTRODUCTION The main objective of work package 5 (WP5) of the STS-Med project is to develop reliable, low-cost, and flexible small/medium multi-generative CS power plants to satisfy the energy demand of buildings in the Mediterranean area.
This technical objective is achieved combining and optimizing innovative subsystems for the efficient capture and conversion of solar energy to essential energy products and services for building: electrical power, air conditioning and water treatment (e.g. desalination or sterilization). The primary solar energy sources can feed this conversion process either alone or in combination with backups (e.g. gas or biomass).
In a first phase of the development, the basic elements to be integrated have been identified, according to the technological demand and specifications of the process (i.e. the capture, conversion and delivery of solar energy). This work is reported in Deliverable D5.1, representing an extensive overview of the different unit options for the plant components:
1. solar concentrators, i.e. the collectors with all the tools to capture the solar radiation and convert it into “high temperature” heat
2. heat transfer fluids (HTF) used to transfer the thermal energy between the units
3. thermal energy storage (TES) systems
4. electric power generation systems (often called “power block”)
5. heating and cooling cogeneration systems for the building
6. desalination and water treatment units
It is noteworthy that Deliverable D5.1 represents a first step for the production of a “handbook” for the small-medium CS technology, i.e. a sort of catalogue of technological and commercial options as a tool designers and installers. Therefore, this handbook will result from the combined outcome from WP5 (Task 5.1) and the components definition in WP6 (Task 6.1), and will represent a major WP5 outcome to be capitalized at the end of the STS-Med project.
The next step of the development consists on the optimization of the overall process: in order to obtain reliable, low-cost, and flexible small/medium multi-generative CS power plants, the best combination of the above mentioned components should be determined in order to satisfy the energy needs of buildings in the Mediterranean area.
This optimization study implemented in the framework of Task 5.2 “Choice of the optimal combination/integration of technologies” is presented in this report.
2. System integration and optimization A general representation of the multi-generative STS-Med plant concept is shown in Figure 1. Each unit is represented as a “black box” which needs to be characterized, based on the end users’ demand, the site characteristics, and the available components on the market.
The solar collector of the CS plant transfers the heat captured from solar radiation in the solar field to the final users by means of a Heat Transfer Fluid (HTF).
The core and central knot of the system is represented by a Thermal Energy Storage (TES) system. Indeed, as explained in the following section of this report, the application of a suitable Thermal Energy Storage (TES) systems could make the CS technology appealing and competitive with other Renewable Energy technologies (e.g. PV or wind): in principle, reliable TES systems allow easer management of the produced energy, increase the “capacity factor” of the plant and allow mismatch of between the fluctuating primary source (solar
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energy) and the final user (the building). Therefore, in order to optimize the system it is recommended to install a TES in the plant, with a specified heat capacity.
The produced energy can simultaneously or alternatively drive the following heat demanding sub-systems:
• power block (e.g. an ORC, steam micro-turbine Rankine cycles, etc.) for electrical power production;
• air conditioning for building heating/cooling;
• water treatment units.
Figure 1. General block diagram of the integrated STS-Med multi-generative CS plant.
As for the overall plant layout, the above-mentioned heat demanding sub-systems can be connected either in series or in parallel with respect to the TES.
When the heat demanding units are connected in series, as represented in Figure 2, only one discharge heat exchanger is directly coupled with the TES. In this case, the power block is usually the first connected heat load since thermo-electrical cycles usually require heat at higher temperatures; the co-generative power block will feed the process heat for the other units in cascade.
Alternatively, the heat demanding units can be connected individually to the TES, by means of an individual heat exchanger or, most likely, by individual valves directing the heat exchange fluid from a single heat exchanger to the units individually. This last configuration allows the application of heat demanding units to work independently each other, thus maximizing the flexibility of the whole system to follow the different energy demands (power, air conditioning, water).
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Figure 2. Block diagram of the multi-generative CS plant with heat demanding units (in this case a MED desalination unit and a chiller for cooling) connected in series by means of a co-generative power block (in this case an ORC).
Some small/medium CS plants involve the direct electric power production integrated in the solar collector, with a solar receiver specifically designed for direct power production, as shown in Figure 3. This is the case, for example, of Concentrating Photovoltaic (CPV) and dish collectors with Stirling engines or micro-turbines. These systems can be also conceived and designed in a co-generative mode, with the collected thermal energy that will directly drive the heat demanding units. In these systems, however, the electric power production does not take advantage of the thermal energy storage benefits to mismatch the power demand from the solar source.
Figure 3. Block diagram of the multi-generative CS plant with direct electric power production from solar collectors.
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The basic data for performance evaluation and optimization in the design phase is the solar radiation level and weather conditions of the reference plant site. Additionally, since the plant should provide electric power, cooling and water treatment services for the building(s), the effective and relative ratio between these duties should be made available in the form of “energy audits”: as illustrated in Figure 4, once the operational range of the integrated sub-systems (T1, T2, T3, …) will be identified, their matching with adaptable loads (C1, C2, C3, …) can be identified.
Figure 4. Matching between the operational characteristics of the integrated sub-systems and the heat and power demand from the building.
Figure 5 shows examples of monthly energy demand (heat and electrical power) by two different public buildings in Sicily: the relative heat and power duties of the building largely changes on either seasonal and daily basis, so it is recommended to design multi-generative CS plants with high flexibility. Plant architectures with parallel connection of the sub-units as shown in Figure 1 (with respect to the layout in series with the co-generative power block, as in Figure 2) will ease flexibility of operation and adaptation to the heat and power loads.
Then, the best compromise between investment costs, easy management, compactness, reliability and flexibility should be key criteria for the selection in the optimization phase.
A number of combination/integration of sub-systems (the “toolbox components”) can be selected, and suitable simulation tools and models applied to evaluate the integrated systems and identify the “best” combination/integration of the sub-systems, also considering the input from work package 4 (Task 4.3).
Clearly, it is necessary to ensure a high level of integration of different components in a range of applications and structured environments. The selection of the optimal plant layout and the most favourable combination of components depends on the specific energy demand by the final user (building), the site characteristics (e.g. available surface for the solar collectors, solar radiation trends, etc.) and the market availability/constraints (and costs) of the selected components. Each configuration can be analysed and assessed following a specific methodology, as reported in Deliverable D5.3, to be applied in multi-generative use-cases.
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Figure 5. Examples of energy demand (heat and electrical power) by different public buildings in Sicily and in Egypt: top diagrams refer to two public buildings in Sicily, bottom diagram refer to a medical center in Egypt in the summer (left) and winter (right).
The characteristics of the available technology solutions to be integrated are described in Deliverable D5.1.
In some cases the proposed components for small-medium scale multi-generative CS plants are commercial but still in an “early market” stage. Therefore, the techno-economical optimization should consider the cost reduction effects resulting from the mass production of components today produced only for niche markets. This is the case, for example, of the mini-Fresnel collectors and the ORC systems for power production.
Additionally, optimized integrated CS systems might require innovative solutions for components not yet commercially available, yet, but still in a R&D or RD&D stage with a limited maturity or “Technology Readiness Level” (TRL). This is the case, for example, of low cost and easy-to-handle Thermal Energy Storage (TES) systems.
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2.1 Optimization of the solar field The solar field consists of the solar collectors with solar tracking system, solar receivers, and the piping to distribute/collect the Heat Transfer Fluid (HTF) from/to the Thermal Energy Storage (TES) system and the heat load exchangers. All the components are by themselves commercial products and, therefore, the optimization aims to:
1. reduce the costs and increase the performance of the solar field;
2. identify the best compromise between the size of the solar field and the utilization of back-up fuel for hybridization during periods with lack of solar radiation;
3. improve the integration with the surrounding environment and the building(s).
The cost of the solar field can be minimized with a simplified design of the collectors and the tracking system. It is expected that the unit cost for small CS collectors can be reduced to less than 200-250 €/m2 when mass production will be achieved after the installation of several plants. Moreover, small linear Fresnel collectors allow applying a “light” modular structure reducing the civil works costs for the foundations and structures necessary to fix the collectors to the ground and withstand wind loads.
The utilization of evacuated receivers (with maximum absorption efficiency of the concentrated solar radiation) and the optimization of optics and mirror cleaning procedures will also allow improvement of the overall solar-to-heat efficiency.
As far as the plant sizing is concerned, there is a relationship between the solar field active area (m2), the thermal energy storage capacity (MWh thermal) and the amount of a back-up fuel (e.g. natural gas or biomass) to be applied to grant the necessary thermal power. Considering today’s incentives and financial contributions usually issued for renewable energy production, the back-up fuel rate is expressed as “percentage” (%) of the cumulative yearly energy produced. Figure 6 shows an example of this design function: for a given site (i.e. solar radiation) and a solar field area, if no TES system is applied there can be a number of hours per year when the collected solar heat will exceed the user(s) demand (see next section 2.3 and Figure 10) so the solar collectors should be “defocused” and a high back-up rate should be applied to extend the power production during the “non-solar” periods. Hence, the application of energy storage will significantly reduce the back-up rate, and there will be a lower threshold for the TES capacity to maximize the capture of the solar radiation (and minimize the back-up fuel consumption).
Figure 6. Example of relationship between solar field active area, heat storage capacity and yearly back-up fuel consumption in a CSP plant (adapted fromM. De Falco, A. Giaconia, et al.; Int. J. Hydrogen Energy, 2009, 34, 98-109).
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Finally, the solar field represents the largest component of the multi-generative CS plant and, therefore, its integration with the surrounding environment and the building(s) is of major concern. Linear Fresnel collectors represent the CS technology with minimum land consumption, i.e. maximum coverage factor of the solar field area (mirrors’ active area by the total land) and, hence, easily adaptable to the available area. Moreover, due to the above-mentioned light structure, the solar field can also be installed on the building roofs. In Figure 7 is provided the picture of the STS-Med linear Fresnel solar field built in Cyprus on the rooftop of a building (the Novel Technologies Laboratory in Nicosia).
Figure 7. STS-Med linear Fresnel solar field built on the rooftop of the Novel Technologies Laboratory in Nicosia (Cyprus) as an example of optimized integration with the building.
The application in the solar field of non-polluting heat exchange fluids (HTF) with minimal toxicity is another measure to improve the integration with the environment. This is the case, for example, of the mineral bio-derived oil as HTF applied in the CS plant built in Palermo: again this is an optimized compromise between the maximum temperature reachable in the solar field output (< 310°C) and the environmental impact of the plant.
2.2 Optimization of the CS plant with thermal energy storage As mentioned in the previous section the introduction of a suitable and reliable Thermal Energy Storage (TES) system in a CS plant makes the whole technology competitive and appealing.
There are several options available CS heat storage (see Deliverable D5.1). In multi-generative CSP plants, where electrical power is produced, the captured heat should be stored at temperatures high enough to drive thermo-electrical cycles with satisfactory efficiency, usually > 200°C (preferably > 250°C). Therefore, HTF and TES systems suitable for such relatively high-temperature ranges should be applied. For this purpose, in principle, there is a rather wide technological offer, exploiting hot fluids as storage material (e.g. molten salts, oil, pressurized steam), Phase Change Materials (PCM), solid concrete storage, and chemical storage systems (e.g. calcium oxide/hydroxide). However, the above-mentioned commonly used TES options are characterized by either a complex management or limited maturity (Technology Readiness Level, TRL < 7 according to the typical EU scale and definition).
In Figure 8 is represented a typical scheme of the TES system applied in conventional CSP plants, making use of molten salts (molten nitrites mixture NaNO3/KNO3 60/40 w/w, often called “solar
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salt”). Accordingly, CSP plants use a two-tank heat storage system. In Figure 8(a) shows the case when the incoming solar radiation (DNI) is satisfactory: the solar collectors rise the temperature of the Heat transfer Fluid (HTF, typically oil) from a given inlet minimum value (Tin, typically around 300°C) to an outlet maximum value (Tout, typically around 390°C). Then, the HTF can directly power the heat load user, typically a steam generator for a steam Rankine cycle for electrical power production. When the available HTF flow rate exceeds the heat load, the extra flow is directed to the TES system by means of a heat exchanger where the extra heat is transferred to molten salts (solar salt) flowing from the lower temperature tank (typically at TS-low = 290°C) to the higher temperature tank (typically at TS-high = 380°C). Therefore, during the sunny hours the heat demand can be satisfied and simultaneously the higher temperature tank of the TES loaded. When the solar radiation is no longer satisfactory to rise the HTF temperature to the nominal value (e.g. after sunset), power production can be maintained by reversing the flow of the HTF and the solar salt between the tanks, as shown in Figure 8(b).
This general TES scheme is widely applied worldwide in large CSP plants with power range of 10-500 MW capacity. For small-medium CS plants (< 1 MW) it is rather difficult to replicate such a complex scheme. This is partially due to the lower temperatures typically involved in small-medium CS systems, and principally due to the need of expert personnel to manage molten salts loops.
Therefore, starting from ENEA’s consolidated background in the molten salts based TES systems, in order to optimize the small-medium CS plants in STS-Med, an innovative TES system has been specifically developed. This system is still based on the use of molten salts, but the management of the TES is eased.
First, a different salt with much lower melting temperature is applied, consisting of the ternary mixture CaNO3/NaNO3/KNO3 (42/15/42 %w): differently from the “solar salt” binary mixture (NaNO3/KNO3 60/40 %w) with melting temperature around 220°C, this mixture is characterized by a melting temperature around 120°C. Hence, the temperature range is much more compatible with the above-mentioned small-medium CS temperatures (> 200°C). Moreover, this material has lower unit cost than the solar salt while keeping the positive features like high heat capacity, low vapor pressure (no pressurized tank is needed), thermal stability (up to 450°C), minimal corrosion rate on carbon steels, no toxicity, no flammability, no relevant environmental impact in case of leakage (the sale salts are commonly used as fertilizers).
Second, the two-tank TES is replaced by a single-tank system avoiding the external pumping of the molten salts and the management of molten salts pipelines. In the developed TES all the typical operations of a CS plant (charging and discharging, as in Figure 8) are achieved inside the single tank where given temperature gradients and molten salts circulation are easily determined. Therefore, besides lower equipment volume and cost reduction potentials, the plant operator should not take much care of pumping and managing molten salts flows. Thus, the developed TES is specifically tailored for civil and residential users and fits with the STS-Med requirements.
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Figure 8. Example of a TES system in a CSP plant, during the daily TES charging phase (a) and the TES discharging (e.g. night) phase (b).
A scheme of the developed TES system during the charging and discharging phases is represented in Figure 9. The operation concept is based on the properties of unmixed molten salts in the tank to stratify in temperature along the vertical axis, as an effect of their low thermal conductivity and the density variability with temperature. Two heat exchangers are immersed in the zones where the temperature is lower (bottom) and higher (top) to be operated during the charging and discharging phases.
(a)
(b)
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Figure 9. Optimized TES system developed for STS-Med: general scheme with explanatory working conditions (left) and prototype drawings (right).
A small TES prototype has been designed, built, installed and tested at ENEA-Casaccia research center in order to validate the concept before installation in the pilot plant in Palermo. This prototype, characterized by an inner volume around 0.9 m3 (corresponding to effective heat capacity of 50-170 kWh) has been successfully tested and some experimental results are presented in Deliverable D5.4.
Further optimizations and improvements have been made in a “new” version of the TES to be installed in the STS-Med pilot plant in Palermo, with an inner volume around 7.0 m3, corresponding to effective heat capacity of about 400 kWh and charging/discharging thermal power the order of 250/125 kW.
2.3 Energy dispatchability and techno-economic issues Investment costs CSP plants for the electrical power production on large scale (> 50 MW) are usually in the range of 4,000 – 9,000 €/kW (source: IEA “Solar Thermal Electricity – Technology Roadmap”, 2014). High-temperature heat storage systems (TES) possibly combined with plant hybridization using back-up fuels allow to maximize the number of power production hours and, hence, to maximize the utilization of the heat and power demanding units. The cost of the solar collection system (solar collectors, civil works, tracking system, receivers, HTF piping, sensors, etc.) for large scale CSP plants represents about 50-60% of total investment; the TES and power block impacts on the total investment costs are usually within the range of 20-30% and 10-20%, respectively.
Solar fields in CSP plants are sized for a peak thermal power production much higher than the nominal thermal power load of the power block (the ratio is called “solar multiple”) and the extra thermal power produced is stored in the TES system. This concept is schematized in Figure 10: the application of a heat storage system in CSP allows to spread the electrical energy (EE) power production over an extended period and control the production at the nominal rate (or follow the power load).
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Figure 10. The thermal energy storage concept: extension and spreading of the electrical energy (EE) power production (a), and power spreading effects of the TES system, by comparison between CSP and direct solar technologies like PV (b).
Therefore, the yearly operational hours of a CSP plant and overall production (MWh/year) is higher than other renewable energy technologies (e.g. PV): depending on the solar radiation and the TES size (usually designed for 4-16 hours of thermal storage) solar multiple values in the range of 2-3 can be obtained. In other words, the higher installation costs of the CSP technology (with respect to other solar power technologies) can be, to some extent, balanced by the larger annual production of energy. These considerations make the large scale CSP plants attractive, further motivated by the learning rates and predictions about the investment cost reduction after massive increase of worldwide installed cumulative capacity (see: IEA “Solar Thermal Electricity – Technology Roadmap”, 2014; Sargent & Lundy LLC Consulting Group – NREL Subcontractor Report, NREL/SR-550-34440, October 2003).
Clearly, when small-medium scale CS plants are concerned the scenario might be different.
Although small CS collector technology differ from the large one, it is expected that the unit cost (€/m2) of the overall solar collection system (solar collectors, civil works, tracking system, receivers, HTF piping, sensors, etc.) will not significantly differ, being in the range of 200-250 €/m2. Indeed, the smaller collectors might be characterized by higher specific costs, but the lighter structure would reduce the civil work expenses.
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As far as the heat storage system is concerned, Figure 11 shows the trend of the heat capacity (expressed in terms on molten salts volume) versus the maximum temperature of the TES. In a conventional CSP TES system (Figure 8) with the high temperature tank at TS-high = 380°C, about 280 m3 of “solar salts” (NaNO3/KNO3 mixture, 60/40 %w) should be loaded to store 20 MWh thermal energy (i.e. 5 MW thermal dispatched for 4 hours), to drive a steam Rankine cycle. The same heat storage capacity, in principle, can be obtained applying the STS-Med TES system (section 2.2) with maximum temperature of 300°C, combined with an ORC, and loaded with about 180 m3 of ternary salt (CaNO3/NaNO3/KNO3, 42/15/42 %w).
Figure 11. Comparison between the heat storage applied in conventional CSP plants (two-tanks TES with solar salt) and the TES system developed in STS-Med project in terms of molten salts volume and temperatures. Reference case: 4 hours storage for 5 MW thermal load (i.e. 20 MWh thermal).
Considering that the reduction of the overall amount of salt, the lower cost of the ternary mixture (compared to the solar salt), the use of a single tank (in place of the two-tanks system), the avoidance of external molten salt pumps and pipelines, it is expected that the cost (€/kWh thermal) of the optimized heat storage system developed in STS-Med project will not be higher than the large scale CSP benchmark (Figure 8).
It is also expected that the TES system developed in STS-Med, if further optimized for medium scale applications (1-10 MW CSP plants) can approach the international target for CSP storage of 15 €/kWh (thermal).
The major difference between small-medium and large CSP plants, in terms of components cost impact, is the power block: the integration of the electric power production systems in small-medium CS plants should be optimized, either in terms of efficiency and costs (€/kW electrical). Today’s typical values of thermo-electric efficiency of large CSP plants are in the range of 30-40%, and the cost of the power block (steam generator, turbine, heat exchangers, etc.) is usually less than 1000 €/kWe. Differently, ORC (or steam micro-turbines) in the 0.1-1 MWe range are characterized by much lower efficiencies, the order of 10-24%, (see Deliverable D5.1) and much higher installation costs. Roughly, less similar installation costs (€/kWe) lead to less than half electrical power production in small-medium power blocks.
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The application TES systems to extend and smooth the electrical power production, as shown in Figure 10, can mitigate the cost impact of the power block, since smaller units can be installed for equivalent daily (or yearly) electrical power production. However, reducing the size of the unit does not always lead to major cost benefits, and in some cases weakens the efficiency and flexibility of the system.
The optimized solution is represented by co-generation and multi-generation systems, where part of the generated heat is directly supplied to satisfy the heat demand of the buildings, for services usually directly powered by electrical energy or fossil fuels. As discussed in section 2, it is preferable to connect the direct heat demanding units in parallel with the TES (as shown in Figure 1) in order to maximize the flexibility of the heat and power generation system and satisfy the daily and seasonal variations in the duties (Figure 5). This design concept is most reasonable when the expected electrical power vs. heat demand of the building exceeds the generation value with a co-generative power block (Figure 2). In these cases, in order to fit the production with the demand, it is recommended to consider optimized solutions involving the integration of the small-medium CS plant with PV or CPV systems, to provide the extra electrical power by the user(s).
3. SUMMARY AND CONCLUSIONS This report provides an assessment of the small-medium CS multi-generative technology, tracing the optimization guidelines. This work has been implemented in the framework of work package 5, Task 5.2 “Choice of the optimal combination/integration of technologies”.
Specifically, after the identification of the basic components (Deliverable D5.1) the next step of the development consisted on the optimization of the overall process: in order to obtain reliable, low-cost, and flexible small/medium multi-generative CS power plants, the best combination of the above mentioned components has been determined to satisfy the energy needs of buildings in the Mediterranean area.
It was assessed that the Thermal Energy Storage (TES) system represents the core and central knot of the plant. Indeed, the application of a suitable TES could make the CS technology appealing and competitive with other Renewable Energy technologies.
In this report it is highlighted that some toolbox components (e.g. mini-Fresnel collectors and the ORC systems) are commercial but still in an “early market” stage: cost reduction effects resulting from the mass production of components today produced only for niche markets should be considered in the techno-economic assessment.
Moreover, optimized integrated CS systems might require innovative solutions for components not yet commercially available, yet, but still in a R&D or RD&D stage with a limited Technology Readiness Level (TRL). In particular, TES systems suitable for such relatively high-temperature ranges should be applied, but commonly used TES options for large CSP plants are characterized by either a complex management or limited maturity. Therefore, an innovative TES system has been specifically developed for STS-Med project: similarly to conventional TES systems, the developed TES technology is still based on the use of molten salts, but its management simplified so that the developed TES is specifically tailored for civil and residential users and fits with the STS-Med requirements. A small TES prototype has been designed, built, installed and tested at ENEA-Casaccia research center in order to validate the concept before installation in the pilot plant in Palermo. This unique prototype has been successfully tested and some experimental results are presented in Deliverable D5.4. Further optimizations and improvements have been made in a “new” version of the TES to be installed in the STS-Med pilot plant in Palermo.
As for the overall plant layout, the heat demanding sub-systems (water treatment and building heating/cooling unit) can, in principle, be connected in series or in parallel with respect to the TES.
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Considering that pant flexibility should be prioritized, the connection in parallel has been evaluated more convenient.
Since small-medium scale CSP plants (with electrical power production) are not commercially widespread yet, a techno-economic benchmarking comparison with large CSP plants has been presented. The benchmark (conventional) solution is represented by large scale CSP plants (> 10 MW) using oil heat transfer fluid in the solar field, two-tank heat storage system with solar salts in the temperature range of 290-380°C, connected with steam Rankine cycles for power production. In the small-medium CS plants considered in STS-Med are the maximum temperatures of the oil heat transfer fluid in the solar field are usually less than 300°C; the above mentioned single tank storage and ORCs have been often selected for this application. The toolbox components with higher impact and optimized solutions have been identified.
The results of this assessment recommend the implementation of optimized multi-generative systems. Besides parallel connection of the “direct heat” demanding units (e.g. desalination units, chillers, etc.), in order to best fit the production with the demand, it is recommended to consider optimized solutions involving the integration of the small-medium CS plant with PV or CPV systems, to provide the extra electrical power by the user(s).
Partnership
• The Cyprus Institute (Cyprus)
• Cyprus Chamber of Commerce and Industry (Cyprus)
• cade or cienti�ic esearch and Technolo pt• ew and enewable ner uthorit pt• lsewed lectric pt• rench lternati e ner ies and to ic ner o ission rance• ro ence nno ation business support rance• ni ersit o thens nstitute o cceleratin ste s and pplication reece• l al a pplied ni ersit ordan• inistr o ner and ineral esources ordan• illeniu ner ndustries ordan• icil e ion epart ent o roduction cti ities tal• ational enc or ew Technolo ies ner and ustainable cono ic
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