Thermal Storage Size Research Articles

Limitations of using LCOE as economic indicator for solar power plants

With the increasing penetration of renewable energy, the importance of power plant dispatchability has become more relevant; an aspect that is not accounted in the Levelized Cost of Electricity (LCOE) parameter. This paper presents a mathematical model to optimize solar plant design based on optimized dispatching strategies considering real grid demand and prices. To demonstrate that the LCOE is not the correct optimization parameter of Photovoltaic and Concentrated Solar Power technologies, the application of the new approach is performed considering their installation in South Australia and Southern California. Sixteen different cases are proposed to assess the importance of the boundary conditions (location with corresponding electricity load curve, cost of the technology and possibility to purchase electricity from the grid) on the plant design.Results show that the optimal plant design significantly differs when considering the actual prices with respect to the common LCOE approach. The LCOE approach does not lead to any storage installation in PV plants and operates CSP plant around the clock. The optimized design based on the grid needs generates the electricity when the prices are high by introducing the storage in the PV and reducing the solar field area as well as the thermal storage size. Overall, the Net Present Value and the IRR can be increased by 10 times and 70 %, respectively, by adopting optimized design with respect to the one achieved with the typical LCOE approach.

Assessing parabolic trough collectors and linear Fresnel reflectors direct steam generation solar power plants in Northwest México

Concentrating solar power (CSP) systems offer promising solutions for harnessing solar energy. Parabolic trough collectors (PTC) are prevalent in CSP, but direct steam generation (DSG) in solar fields is an innovative alternative that eliminates the need for thermal oils. This study compares EuroTrough PTC and optimized linear Fresnel reflector (LFR) geometries for DSG for Mexican operating conditions. Two 10 MW Rankine cycles with two and three steam extractions are considered. The study evaluates 16 solar field configurations capable of delivering 400 °C superheated steam at 100 bar. Assessment parameters include effective concentration area, nominal loop inlet pressure, energy and exergy efficiency, and hypothetical thermal storage size. The findings strongly favor PTC solar fields with 14 loops, exhibiting superior performance. However, three-loop LFR configurations are suggested where pressure reduction prevails. Three-loop LFR arrays minimize land utilization for spatially constrained installations, while larger five-loop configurations optimize efficiency. Employing 14 PTC loops prioritizes thermal energy storage. In contrast, 8 PTC loops maximize storage capacity. The study underscores the advantages of Fresnel reflectors over parabolic troughs in specific scenarios, presenting avenues for future exploration. This analysis lays the groundwork for assessing these technologies in the distinctive context of Mexican operations, offering valuable insights into sustainable energy applications.

Scale-Up Considerations of the Sundial Rotating Fresnel Solar Collector

In the frame of the ASTEP project two prototypes of the Sundial collector, together with a Phase Change Material Thermal Energy Storage, will be tested integrated with the industrial processes of two sites. However, the size of the pilot systems will not be optimized for commercialization. The present work considers the scale-up of the concept for one of the pilot industrial sites: a dairy industry located in Greece, and how will the thermal losses of the system must be considered with the increased aperture, thermal storage size and energy output. For the scaling up of the system, the capacities of the Sundial, demand and TES were doubled and multiplied by 10, and the thermal losses of both nominal conditions and annual performance were considered. The results showed that the ratio between the piping heat losses and the total available energy decreased when scaling-up the system both in the nominal and in the annual results. Hence, the heat losses lost importance when scaling up, when more energy was available. These facts should be considered when evaluating the performance of the pilots and feasibility of the concept.

On-site solar PV generation and use: Self-consumption and self-sufficiency

As energy storage systems are typically not installed with residential solar photovoltaic (PV) systems, any “excess” solar energy exceeding the house load remains unharvested or is exported to the grid. This paper introduces an approach towards a system design for improved PV self-consumption and self-sufficiency. As a result, a polyvalent heat pump, offering heating, cooling and domestic hot water, is considered alongside water storage tanks and batteries. Our method of system analysis begins with annual hourly thermal loads for heating and cooling a typical Australian house in Geelong, Victoria. These hourly heating and cooling loads are determined using Transient System Simulation (TRNSYS) software. The house’s annual hourly electricity consumption is analysed using smart meter data downloaded from the power supplier and PV generation data measured with a PV system controller. The results reveal that the proposed system could increase PV self-consumption and self-sufficiency to 41.96% and 86.34%, respectively, resulting in the annual imported energy being reduced by about 74%. The paper also provides sensitivity analyses for the hot and cold storage tank sizes, the coefficient of performance of the heat pump, solar PV and battery sizes. After establishing the limits of thermal storage size, a significant impact on self-efficiency can be realised through battery storage. This study demonstrates the feasibility of using a polyvalent heat pump together with water storage tanks and, ultimately, batteries to increase PV self-consumption and self-sufficiency. Future work will concentrate on determining a best-fit approach to system sizing embedded within the TRNSYS simulation tool.

Techno-economic and GHG mitigation assessment of concentrated solar thermal and PV systems for different climate zones

In this study, energy production by two solar energy technologies, namely concentrated solar power (CSP) and photovoltaic (PV) power, is compared from a technical, economic and environmental perspective. Initially, a 50 MW CSP plant is modeled and simulated at four selected sites in Pakistan. Then, the most feasible location of the CSP plant is compared with the solar PV plant of the same capacity. The effect of the solar thermal storage size and cooling system of the CSP system is investigated, while the photovoltaic tracking system is investigated to evaluate the technical and economic performance of the power plants. Technical performance is evaluated based on energy generation and capacity factors metrics, while economic performance is evaluated with respect to levelized cost, payback period and net present value. In addition, environmental criteria such as reducing greenhouse gas emissions (GHG), saving fossil fuels, and life-cycle water consumption are evaluated. From the results, it was concluded that the CSP plant located in Quetta is technically and economically viable. The capacity factor of the CSP plant is 36.6% compared to 19.8% for the PV plant, while the solar-to-electrical efficiency of the CSP plant is 14.2% compared to 20.8% for the PV plant. The required land area is 2.77 acres/GWh for the CSP plant and 2.33 acres/GWh for the PV plant, while the net capital cost of the CSP plant is five times higher than that of the PV plant. Various design parameters are optimized to obtain the minimum levelized cost of energy (LCOE) for both CSP and PV plants. The results of CSP and PV plants indicate that the LCOE can be reduced to 11.57 cents/kWh and 4.69 cents/kWh, respectively. Thus, the CSP plant performs better from the technical point of view while the PV plant performs better from the economic perspective.

Investigation of Combined Heating and Cooling Systems with Short- and Long-Term Storages

Modern buildings in cold climates, like Norway, may have simultaneous heating and cooling demands. For these buildings, integrated heating and cooling systems with heat pumps, as well as short-term and long-term thermal storage, are promising solutions. Furthermore, combining this integrated system with renewables aids in the transition to future sustainable building energy systems. However, cost-effectively designing and operating such a complicated system is challenging and rarely addressed. Therefore, this research proposed an integrated heating and cooling system that incorporated a short-term water tank and a long-term borehole thermal storage. Meanwhile, three operating modes: heating, cooling, and free cooling were defined based on different heating and cooling load conditions. A detailed system model was developed in MATLAB using heat pump manufacture data as well as simulated and measured building loads. Following that, sensitivity studies were performed to investigate the impacts of ground properties, thermal storage size, setpoint temperature, heat pump characteristics, and load conditions. The findings identified the crucial factors that influence the system’s overall energy efficiency and the functioning of the key system components. Particularly, it revealed that low cooling to heating ratios caused an imbalance in charging and discharging, further reducing the ground temperature and degrading the heat pump’s performance.

Multi-objective optimal design of solar power plants with storage systems according to dispatch strategy

This study presents a comprehensive analysis evaluating the impact of the dispatch strategy on the optimal design configurations of different combinations of solar power plants with storage. The analysis considers four dispatch profiles (baseload, daylight, night, and daylight and evening), and four technology combinations including a solar PV plant with batteries, a CSP plant with Thermal Storage (TES), a hybrid CSP-PV plant with TES, and a hybrid CSP-PV plant with TES and batteries. Two locations with high and moderate levels of DNI were selected and cost scenarios for 2020 and 2030 were considered. The aim is to determine the competitiveness ranges of each technology combination and establishing the least-cost technological option that allows meeting a dispatch strategy with a certain level of supply guarantee. A multi-objective optimization approach was followed to obtain the trade-off curves that minimize the LCOE and maximize the sufficiency factor in terms of the nominal size of the PV plant, solar multiple, TES size, batteries capacity, and inverter power rate. Results of this work allow determining the influence of the dispatch strategy on the competitiveness of these storage-integrated technology options, giving relevant information concerning under which conditions one technology combination is preferable over another.

Optimizing Power and Heat Sector Coupling for the Implementation of Carbon-Free Communities

To achieve a successful integration of fluctuating renewable power generation, the power-to-heat (P2H) conversion is seen as an efficient solution that remedies the issue of curtailments as well as reduces carbon emissions prevailing in the district heating (DH) sector. Concurrently, the need for storage is also increasing to maintain a continuous power supply. Hence, this paper presents a MILP-based model to optimize the size of thermal storage required to satisfy the annual DH demand of a community solely by P2H conversion employing renewable energy. The DH is supplied by the optimal operation of a novel 2-km deep well heat pump system (DWHP) equipped with thermal storage. To avoid computational intractability, representative time steps with varying time duration are chosen by employing hierarchical agglomerative clustering that aggregates adjacent hours chronologically. The value of demand response and the effect of interannual weather variability are also analyzed. Numerical results from a Finnish case study show that P2H conversion utilizing small thermal storage in tandem with the DWHP is able to cover the annual DH demand, thus leading to a carbon-neutral DH system and, at the same time, mitigating the curtailment of excessive wind generation. Compared with the annual DH demand, an average thermal storage size of 29.17 MWh (2.58%) and 13.99 MWh (1.24%) are required in the business-as-usual and the demand response cases, respectively.

Experimental and Numerical Study of a Microcogeneration Stirling Unit under On–Off Cycling Operation

Stirling units are a viable option for micro-cogeneration applications, but they operate often with multiple daily startups and shutdowns due to the variability of load profiles. This work focused on the experimental and numerical study of a small-size commercial Stirling unit when subjected to cycling operations. First, experimental data about energy flows and emissions were collected during on–off operations. Second, these data were utilized to tune an in-house code for the economic optimization of cogeneration plant scheduling. Lastly, the tuned code was applied to a case study of a residential flat in Northern Italy during a typical winter day to investigate the optimal scheduling of the Stirling unit equipped with a thermal storage tank of diverse sizes. Experimentally, the Stirling unit showed an integrated electric efficiency of 8.9% (8.0%) and thermal efficiency of 91.0% (82.2%), referred to as the fuel lower and, between parenthesis, higher heating value during the on–off cycling test, while emissions showed peaks in NOx and CO up to 100 ppm but shorter than a minute. Numerically, predictions indicated that considering the on–off effects, the optimized operating strategy led to a great reduction of daily startups, with a number lower than 10 per day due to an optimal thermal storage size of 4 kWh. Ultimately, the primary energy saving was 12% and the daily operational cost was 2.9 €/day.

An innovative approach to design cogeneration systems based on big data analysis and use of clustering methods

In recent years, collecting energy consumption data has become easier thanks to the decreasing of smart sensors cost. Moreover, the capacity of data analysis using big data methods like machine learning and artificial intelligence has increased. Such methods are expected to be useful to increase the efficiency of energy systems. In this paper, an innovative approach based on big data analysis to design cogeneration systems is presented. More specifically, this study describes how cluster analysis can be applied to analyse energy consumption data. The aim of the method is to design cogeneration systems that can suit energy demand profiles more efficiently, choosing the correct type of cogeneration technology, operation strategy and, if they are necessary, the size of energy storages. In the first part of the paper, the method based on clustering to perform the analysis of the dataset is described. In the second part, a case study based on a cogeneration plant (a wood industry that requires low temperature heat to dry wood into steam-powered kilns) is analysed. An alternative cogeneration system is designed by means of the proposed method in terms of the choice of the cogeneration technology, the sizing of thermal storage, and the operation strategy of the plant. Thermodynamic and economic benchmarks are defined to evaluate the differences between as-is and alternative scenarios. Results show that the proposed innovative method is useful to design cogeneration systems for industry allowing energy and economic savings.

Solar multiple optimization of a DSG linear Fresnel power plant

Linear Fresnel power plants are currently one of the most promising concentrating solar power plants. However there are only a few commercial projects. These power plants have a lower efficiency than parabolic trough collector plants and are still expensive. To increase the efficiency of these plants, the utilization of water/steam in the receivers (direct steam generation, DSG) and thermal storage (TES) have been considered.As case study, a 50MWe solar-only linear Fresnel power plant located in Seville, Spain is considered. The effects of the solar field size and the thermal storage size on the annual production of the plant are analyzed: Nine different solar field sizes and up to eight thermal storage sizes have been compared.An economic optimization is presented in order to determine which plant has lowest Levelized Cost of Electricity (LCOE). It has been found that for the power plants with no-storage the optimum solar multiple (SM) is 1.7, whereas for the cases with thermal storage, the optimum configuration is a larger solar field (SM = 2), with a thermal storage of 2 h.

Multi-objective optimization of district heating and cooling systems for a one-year time horizon

Besides lowering supply temperatures, the concept of fourth generation of district heating (4DH) also includes integration of heating, cooling and power sector. Due to their high interconnectivity, number of involved technologies and relatively long, but at the same time detailed temporal scale, optimization of such systems presents a challenging task. So far, only hourly district heating multi-objective optimization for a whole year period has been carried out, where detailed district heating and cooling multi-objective optimization has been reserved for small scale utilization and short temporal scale, usually covering specific days or weeks. The main objective of this paper was to develop an hourly based multi-objective optimization district heating and cooling model which is capable of defining supply capacities, including thermal storage size, and their operation for a whole year period. The objective functions are minimization of a total system cost, which includes discounted investment and operational costs, and minimization of environmental impact in terms of carbon dioxide emissions. By using multi-objective optimization, this research shows that for equal level of carbon dioxide emissions, combined district heating and cooling systems have lower total discounted cost when compared to district heating and cooling systems which operate separately.

Comparison of CST with different hours of storage in the Australian National Electricity Market

Abstract The recent ratification of the Paris Climate Change Agreement has significant implications for Australia given its emissions intensive economy. It is likely that the electricity sector will need to decarbonize for Australia to meet medium- and long-term emissions reduction targets. This paper explored the potential role of Concentrating Solar Thermal (CST) in a 100% renewable National Electricity Market (NEM) system under different scenarios of CST configuration and subjected the results to sensitivity analysis. A Genetic algorithm (GA) was chosen as the optimization algorithm to seek the least cost combination of renewable generation technologies, transmission interconnectors and storage capacity in the NEM system at hourly temporal resolution. The main finding is that the scenario where all three CST configurations (six, nine, and 12 h of thermal storage) can be deployed achieves a lower system cost than scenarios where the size of thermal storage coupled with CST is limited to one option. The results are sensitive to assumptions of the discount rate, renewable resource availability, and the cost of CST technology. This paper found that meeting demand during winter evenings is the most challenging time period for a 100% renewable NEM power system.

Study and Optimization of Design Parameters in Water Loop Heat Pump Systems for Office Buildings in the Iberian Peninsula

Water loop heat pump (WLHP) air conditioning systems use heat pumps connected to a common water circuit to fulfill the energy demands of different thermal zones in a building. In this study, the energy consumption was analyzed for the air conditioning of an office building in the typical climate of four important cities of the Iberian Peninsula. The energy consumption of one water loop heat pump system was compared with a conventional water system. Two design parameters, the range in the control temperatures and the water loop thermal storage size, were tested. Energy redistribution is an important advantage of the WLHP system, but significant savings came from high efficiency parameters in the heat pumps and minor air flow rates in the cooling tower. The low thermal level in the water loop makes this technology appropriate to combine with renewable sources. Using natural gas as the thermal energy source, a mean decrease in CO2 emissions of 8.1% was reached. Simulations showed that the installation of big thermal storage tanks generated small energy savings. Besides, the total annual consumption in buildings with high internal loads can be reduced by keeping the water loop as cool as possible.

Quantifying the operational flexibility of building energy systems with thermal energy storages

The increasing share of fluctuating renewable energy generation in the energy system increases the need for flexibility options. Building energy systems (BES) with their corresponding thermal energy storages (TES) can be one option for supplying flexibility. To use this option efficiently, a framework to quantify the flexibility of the BES is necessary. It is found that the flexibility of a BES can hardly be described with one single flexibility indicator. Therefore, this paper develops a method to analyze the flexibility of BES in terms of time, power and energy. Different influencing factors are considered, like the heat generator and the thermal storage size. Additionally, the option to aggregate the different flexibility measures on a city district level is addressed. This is necessary as single buildings have a minor impact on higher level energy systems. Finally, a comparison to other flexibility options like battery storage is discussed.

Economic Feasibility of Flat Plate vs Evacuated Tube Solar Collectors in a Combisystem

The aim of this research is to determine the economic feasibility of a solar thermal system used for Domestic Hot Water and Radiant Floor Heating. A two floor house is modeled to create a thermal load. The system design and thermal analysis is studied using TRNSYS. The technical-economic analysis is performed using Microsoft Excel. The optimal type/number of solar thermal collectors and thermal storage size were determined based on the economic figures. The optimum system configuration for the case of evacuated tube system resulted in 8 collectors using a storage relation of 40 L/m2 whereas flat plate system resulted in 12 collectors using a storage relation of 50 L/m2. The return on investment for the flat plate system was calculated in 9 years and the evacuated tube system resulted in approximately 11 years.

Large-scale urban renewable electricity schemes – Integration and interfacing aspects

Large-scale renewable electricity schemes (RES) for urban areas are investigated here with emphasis on RES integration and interfacing with the energy system to limit the outflow of RES electricity outside the city boundaries. Different electricity-to-thermal energy strategies to supply also heating and cooling demands were investigated to enable higher than usual RES shares through converting surplus renewable electricity into thermal energy which often constitute a major part of the end-use energy in a city. Case examples from a southern climate (Shanghai) with distributed PV schemes and a northern city (Helsinki) with off-shore seaside wind power were analyzed. The results from the Shanghai case indicate that when using thermal storage options, 2-folding of the PV capacity from the self-use limit of PV could be possible. With optimal electricity-to-thermal energy strategies and energy management, a 40–65% solar fraction of yearly electricity could be possible. In the Helsinki climate with wind power coinciding with the seasonal load peak, RES may satisfy beyond 30% of all energy and beyond 70% of all electricity demand annually. The size of thermal storage needed in the above cases is between 10 and 100GWh. A higher RES use leads to higher spatial power flows and power variability in the city which requires more sophisticated control strategies and flexible auxiliary power options to handle transmission bottlenecks.

Optimum thermal storage sizing in building services engineering as a contribution to virtual power plants

This article exemplifies a simulation-based approach to finding an optimum size of thermal storage, taking into account virtual power plants. The basic idea is to use thermal storage devices linked to chiller plants to shift the cooling load of a building in order to be able to participate in both real-time pricing markets and balancing energy auctions. Therefore, an existing cooling plant is modelled using the simulation program TRNSYS. To take advantage of volatile energy rates, a simple price-sensitive control strategy has been implemented. Furthermore, the overall-cost function of the cooling plant is linked to the optimization program GenOpt. The simulation determines the optimum storage tank size, which yields minimum overall costs. This article provides the results of a single case study directed at the real-time pricing market European Energy Exchange ‘EEX’ and Germany's balancing energy auction ‘Minutenreserve’. Beyond, some general conclusions have been pointed out so far.

Inlet Air Cooling Applied to Combined Cycle Power Plants: Influence of Site Climate and Thermal Storage Systems

Abstract The paper presents the results of an investigation on inlet air cooling systems based on cool thermal storage, applied to combined cycle power plants. Such systems provide a significant increase of electric energy production in the peak hours; the charge of the cool thermal storage is performed instead during the night time. The inlet air cooling system also allows the plant to reduce power output dependence on ambient conditions. A 127MW combined cycle power plant operating in the Italian scenario is the object of this investigation. Two different technologies for cool thermal storage have been considered: ice harvester and stratified chilled water. To evaluate the performance of the combined cycle under different operating conditions, inlet cooling systems have been simulated with an in-house developed computational code. An economical analysis has been then performed. Different plant location sites have been considered, with the purpose to weigh up the influence of climatic conditions. Finally, a parametric analysis has been carried out in order to investigate how a variation of the thermal storage size affects the combined cycle performances and the investment profitability. It was found that both cool thermal storage technologies considered perform similarly in terms of gross extra production of energy. Despite this, the ice harvester shows higher parasitic load due to chillers consumptions. Warmer climates of the plant site resulted in a greater increase in the amount of operational hours than power output augmentation; investment profitability is different as well. Results of parametric analysis showed how important the size of inlet cooling storage may be for economical results.

A Method of Evaluating Solar Heat Storage in Livestock Buildings

ABSTRACT -AIR type solar collectors for use in livestock buildings preheat the continuous ventilating air. The excess collected heat is often stored in a thermal mass for later release to the building. The performance of a collector/ storage system is usually measured in terms of reductions in supplemental heat during the heating season. Monsen et al. (1981) proposed a design method to estimate the size of thermal storage for an active solar collector in human housing. He defined two extreme hypothetical cases: (a) zero thermal storage, ZTS, and (b) infinite thermal storage, ITS. The supplemental heat demand was estimated as a fraction of ZTS. Monsen et al. (1982) then extended the ZTS/ITS method of design and analysis to a passive solar Trombe wall. The objective of the present work was to develop the ZTS/ITS method of analysis for livestock buildings and test the method for a Trombe wall solar collector/storage system in a swine building.