Optimization study on a solar-assisted air source heat pump system with energy storage based on the economics method

Currently, hybrid renewable energy systems with thermal energy storage have various advantages and are widely used. This paper investigated the performance of a solar-assisted air source heat pump system with energy storage (SASHPS-ES) in Beijing, China, and proposed an optimal operation mode based on economic evaluation. The results indicate that with the optimal heat storage ratio of 50%, the rated capacity of the air source heat pump (ASHP) of the SASHPS-ES system can be reduced by 16.7%, decreasing its annual total cost by 26.5% under a peak-valley electricity price policy. The price of 620¥/m² is critical for the solar collectors. The economics of SASHPS-ES is better than that of an air source heat pump system with energy storage (ASHPS-ES) when the price of the selected solar collector units is less than this critical price (without subsidies from the government). In the current local market, the promotion of SASHPS-ES systems and other solar energy applications requires government subsidies for a period of time. The results can guide the utilization and popularity of SASHPS-ES in China.     

System performance and economic analysis of solar-assisted cooling/heating system

The long-term system simulation and economic analysis of solar-assisted cooling/heating system (SACH-2) was carried out in order to find an economical design. The solar heat driven ejector cooling system (ECS) is used to provide part of the cooling load to reduce the energy consumption of the air conditioner installed as the base-load cooler. A standard SACH-2 system for cooling load 3.5 kW (1 RT) and daily cooling time 10 h is used for case study. The cooling performance is assumed only in summer seasons from May to October. In winter season from November to April, only heat is supplied. Two installation locations (Taipei and Tainan) were examined.It was found from the cooling performance simulation that in order to save 50% energy of the air conditioner, the required solar collector area is 40 m² in Taipei and 31 m² in Tainan, for COP_j = 0.2. If the solar collector area is designed as 20 m², the solar ejector cooling system will supply about 17–26% cooling load in Taipei in summer season and about 21–27% cooling load in Tainan. Simulation for long-term performance including cooling in summer (May–October) and hot water supply in winter (November–April) was carried out to determine the monthly-average energy savings. The corresponding daily hot water supply (with 40 °C temperature rise of water) for 20 m² solar collector area is 616–858 L/day in Tainan and 304–533 L/day in Taipei.The economic analysis shows that the payback time of SACH-2 decreases with increasing cooling capacity. The payback time is 4.8 years in Tainan and 6.2 years in Taipei when the cooling capacity >10 RT. If the ECS is treated as an additional device used as a protective equipment to avoid overheating of solar collectors and to convert the excess solar heat in summer into cooling to reduce the energy consumption of air conditioner, the payback time is less than 3 years for cooling capacity larger than 3 RT.     

Experimental investigation and a dynamic simulation of the solar-assisted energy-storaged heat pump system

In order to improve the performance of the solar-assisted and energy-storaged heat pump system, an experimental setup was constructed. In this study, the solar-assisted energy-storaged series heat pump system and other conventional heat pump systems with no energy storage (series and parallel heat pump systems) are experimentally investigated and compared. The experiments were made in July, August, September, October, November, and December in 1990, under the clear-sky conditions for three heat pump systems. The experimentally obtained results are used to calculate the collector efficiency n_k, heat pump COP, and system COP_sys (coefficient of performance). On the other hand, a dynamic simulation program has been developed for a solar-assisted and energy-storaged heat pump system. The experimental results were compared with the dynamic simulation results.     

Process simulation and techno-economic assessment of a zero liquid discharge/multi-effect desalination/thermal vapor compression (ZLD/MED/TVC) system

Brine from desalination plants is an upcoming environmental threat to aquatic ecosystems. Multi-effect distillation with thermal vapor compression (MED-TVC) is proposed to treat desalination brine solutions of more than 70 000 mg/L of total dissolved solids (TDS). To achieve this, an integrated techno-economic model consisting of three submodels (scaling prediction, mathematical, and economic) is developed and a techno-economic assessment of a 10 m³/d MED-TVC system is presented under two different scenarios. In this respect, various sensitivity analyses were performed, revealing that the 4-effect MED-TVC system operating at 120°C feed steam presents the lowest freshwater cost (US$3.00/m³) and the lowest payback period (3.04 years) under the first scenario, whereas the 2-effect MED-TVC system operating at 120°C feed steam presents the lowest freshwater cost (US$1.69/m³) and the lowest payback period (1.71 years) under the second scenario of waste heat utilization. Exergy analysis for these optimal systems revealed that the exergy efficiency of the optimal system in the second scenario (4.36%) is higher than in the first scenario (4.21%). For both optimal systems, the exergy destruction in the TVC unit and in the effects accounts for more than 79% of the total exergy destruction. Moreover, it was found that thermal energy costs contribute significantly to the costs and affect the design procedure. Scaling up the optimal systems for freshwater production by more than 190 m³/d, freshwater cost becomes constant and can be reduced by up to 50% with waste heat integration. Considering the high quality of the freshwater produced, MED-TVC process can be profitable due to the revenue from the sale of the freshwater. Overall, the results suggest that the MED-TVC process for brine treatment is economically feasible.     

Feasibility study of a solar-assisted heating/cooling system for an aquatic centre in hot and humid climates

This paper reports on a feasibility study of a solar-powered heating/cooling system for a swimming pool/space combination in a tropical environment. The system utilizes an absorption chiller and a cooling tower to meet the facilities and locker room load. The heating is accomplished by employing hot water generated by heat exchange with the solar collector working fluid. Two thermal storage tanks were employed for the collector and domestic use. The absorption chiller utilizes hot water to regenerate the LiBr solution. The proposed system enables the swimming season to be extended from sixteen weeks to fifty-two weeks. Simulation results indicate that a combination of a double glazed collector area of 600–4800 m² and a storage tank volume of 11·36 m³ results in a 25–37% reduction in the consumption of natural gas. Economic analysis is performed based on the life-cycle-cost method and takes into account the effects of discount rate, fuel price and fuel inflation rate. Different scenarios for which the solar-assisted system is economical are presented and analysed. © 1997 John Wiley & Sons, Ltd.     

CO2 payback–time assessment of a regional-scale heating and cooling system using a ground source heat–pump in a high energy–consumption area in Tokyo

We present an assessment of installing a regional heating and cooling system in the Nishi(West)-Shinjuku area of Tokyo, Japan. In this assessment, we estimate the CO₂ payback–time, when air source heat–pumps (ASHP) are replaced with a ground–source heat–pump (GSHP) system. We calculate CO₂ emissions from transportation of the cooling tower, materials for the underground heat exchanger, and the digging loads and transportation loads incurred when the GSHP system is installed to replace the air source cooling system. The total CO₂ emission from the installation of the GSHP system was estimated to be 67,701t-CO₂, with 87% of the CO₂ emissions resulting from the digging process. CO₂ emissions from the operation of the GSHP system were estimated from the total energy-efficiency of the system and the heating and cooling demand in Nishi-Shinjuku area. Using the GSHP system, 33,935t-CO₂ would be emitted per year. We estimate that using the GSHP system would result in a reduction of 54% of the CO₂ emissions, or 39,519t-CO₂ per year. From these results, the CO₂ payback–time for replacing the conventional ASHP in the 1 km² studied region with the GSHP system is assessed to be 1.7 years.     

Analysis of solar desiccant cooling system for an institutional building in subtropical Queensland,Australia

Institutional buildings contain different types of functional spaces which require different types of heating, ventilating and air conditioning (HVAC) systems. In addition, institutional buildings should be designed to maintain an optimal indoor comfort condition with minimal energy consumption and minimal negative environmental impact. Recently there has been a significant interest in implementing desiccant cooling technologies within institutional buildings. Solar desiccant cooling systems are reliable in performance, environmentally friendly and capable of improving indoor air quality at a lower cost. In this study, a solar desiccant cooling system for an institutional building in subtropical Queensland (Australia) is assessed using TRNSYS 16 software. This system has been designed and installed at the Rockhampton campus of Central Queensland University. The system's technical performance, economic analysis, energy savings, and avoided gas emission are quantified in reference to a conventional HVAC system under the influence of Rockhampton's typical meteorological year. The technical and economic parameters that are used to assess the system's viability are: coefficient of performance (COP), solar fraction, life cycle analysis, payback period, present worth factor and the avoided gas emission. Results showed that, the installed cooling system at Central Queensland University which consists of 10 m² of solar collectors and a 0.400 m³ of hot water storage tank, achieved a 0.7 COP and 22% of solar fraction during the cooling season. These values can be boosted to 1.2 COP and 69% respectively if 20 m² of evacuated tube collector's area and 1.5 m³ of solar hot water storage volume are installed.     

Thermal–economic–environmental analysis and multi-objective optimization of an internal-reforming solid oxide fuel cell–gas turbine hybrid system

In this article, an internal-reforming solid oxide fuel cell–gas turbine (IRSOFC–GT) hybrid system is modeled and analyzed from thermal (energy and exergy), economic, and environmental points of view. The model is validated using available data in the literature. Utilizing the genetic algorithm optimization technique, multi-objective optimization of modeled system is carried out and the optimal values of system design parameters are obtained. In the multi-objective optimization procedure, the exergy efficiency and the total cost rate of the system (including the capital and maintenance costs, operational cost (fuel cost), and social cost of air pollution for CO, NO_x, and CO₂) are considered as objective functions. A sensitivity analysis is also performed in order to study the effect of variations of the fuel unit cost on the Pareto optimal solutions and their corresponding design parameters. The optimization results indicate that the final optimum design chosen from the Pareto front results in exergy efficiency of 65.60% while it leads to total cost of 3.28 million US$ year⁻¹. It is also demonstrated that the payback time of the chosen design is 6.14 years.     

Optimization of a solar-assisted drying system for drying bananas

This paper presents a mathematical model for optimal design of a solar-assisted drying system for drying bananas. The optimization model consists of a simulation model of a solar-assisted drying system combined with an economic model. The simulation model is composed of two systems of differential equations: one for the collector and other for the drying cabinet. These systems of the differential equation were solved using the finite difference method. Values of the model parameters were determined experimentally. A computer program in FORTRAN was developed to simulate the performance of the drying system. The model was validated by comparing the simulation results with the experimental results and they were in good agreement. This simulation model was used for the optimization of the solar-assisted drying system. An economic model was formulated to calculate the annual drying cost. The optimization problem was defined as the optimization of the geometry and operational parameters of the drying system so as to minimize the drying cost per unit of dried product. Currently used collector area and the air recycle factor were considered as the parameters for basic mode of operation of the drying system. The adaptive pattern search technique was adopted to find the optimum values of the solar collector area and the recycle factor. The optimum values of the collector area and the recycle factor were found to be 26 m² and 90%, respectively. The computer program developed in this study can be used to optimize similar drying systems.     

DESIGN AND PERFORMANCE EVALUATION OF A SOLAR POND FOR INDUSTRIAL PROCESS HEATING

This paper describes the design of a solar pond for delivering 54 m³/day of hot water at 60°C to a catering facility in Singapore. The design of the pond was carried out in two steps. First, the depths of different layers of the pond were determined by considering the maximum temperature of the storage zone and the useful energy gain. For the given load and the local meteorological conditions, the optimum depths of various layers of the pond were found as follows:

Depth of surface-mixed layer:0.32 m

Depth of the insulation zone :1.00 m

Depth of storage zone:1.00 m

Total depth of the pond:2.32 m

The minimum payback period was used as the economic figure of merit to determine the optimum area of the pond. The optimum area of the pond is 6000 m². The payback period depends on the transparency of the pond, and for the conditions considered in the study, it varies between 3 and 4.5 years, The solar fraction varies from 65% for extinction coefficient, ( μ = 1.0m^?1 to 94% for μ = 0.55 m^?1, An experimental pond with an area of 14 m² and a depth of 1.5 m was built and tested over a period of time under the meteorological condition of Singapore. These results are used to validate the mathematical equations used in the design of the solar pond. A good agreement was found to exist between experimental and analytical results.     

Improving the effectiveness of solar pumping systems by using modular centrifugal pumps with variable rotational speed

The possibility of improving the performance of deep well solar pumping systems by using centrifugal pumps with variable rotational speed and modular number of working stages (i.e. Divided Shaft Pump, DSP) was investigated and compared with traditional systems equipped with pumps having a fixed number of stages (i.e Standard Centrifugal Pump, SCP). Starting from commercially available pumps with a given head–mass flow characteristic, a visual simulation tool for the evaluation of the modified DSP pump performance and costs was developed. In principle, it would be possible to use the desired number of modular stages, thus achieving the highest efficiency of the system for all conditions of radiation. In practice, in order to reduce the DSP pump costs to an affordable level, only one shaft breakpoint is suggested (and then two modular blocks of stages), whose optimised position is determined by the simulation program on the basis of insolation curve during the daylight and required head and shaft speed.Referring to a 30 m² PV system (about 3000 W peak power) and to a well depth of 100 m and considering a commercial 46-stage submersible pump, it was found that a breakpoint at the 31st impeller produced an increase close to 9% of the yearly pumped water yield with respect to a conventional, non-modular pump.For the above system, assuming that the cost of a modified modular pump is 1.5 times higher than that of a standard pump, the payback time varies from 0.5 to 2.5 years when the water sale price ranges between 1.1 and 0.6 €/m³.     

Long-term performance of solar-assisted heat pump water heater

A long-term reliability test of an integral-type solar-assisted heat pump water heater (ISAHP) was carried out. The prototype has been running continuously for more than 13,000 h with total running time >20,000 h during the past 5 yr. The measured energy consumption is 0.019 kWh/l of hot water at 57 ^oC that is much less than the backup electric energy consumption of the conventional solar water heater.     

A method for technical-economic analysis of solar heating systems

The necessity for technical-economic analysis of solar energy systems is obvious when assessing their feasibility vis-á-vis conventional alternative systems. Optimum magnitudes of the installation parameters should be defined under the required economic conditions. In this study, the optimization procedure was chosen so as to maximize the total accumulated saving throughout the economic lifetime of the system. The annual solar heating fraction of the system is assessed using the f-chart method which can be used for both domestic hot water and space heating. The saving produced by investing in a solar installation is obtained by taking the difference between the total discounted expenditures of the conventional and the solar systems, accumulated during their foreseen lifetimes. To this end, the present value method is applied, taking into account the initial investment costs, fuel costs, operation costs and the maintenance costs for both the solar system and its conventional alternative. Based on this technical-economic analysis, a computer program is developed. This accepts three types of input data: technical design, economic parameters and meteorological conditions, and calculates the optimum magnitudes of the design parameters. It is concluded that economic parameters are much more influential on the system economics than the technical parameters. The most significant are the payback period and the internal rate of return.     

Residual learning rates in lead-acid batteries: Effects on emerging technologies

The low price of lead-acid, the most popular battery, is often used in setting cost targets for emerging energy storage technologies. Future cost reductions in lead acid batteries could increase investment and time scales needed for emerging storage technologies to reach cost-parity. In this paper the first documented model of cost reductions for lead-acid batteries is developed. Regression to a standard experience curve using 1989–2012 data yield a poor fit, with R² values of 0.17 for small batteries and 0.05 for larger systems. To address this problem, battery costs are separated into material and residual costs, and experience curves developed for residual costs. Depending on the year, residual costs account for 41–86% of total battery cost. Using running-time averages to address volatility in material costs, a 4-year time average experience curve for residual costs yield much higher R², 0.78 for small and 0.74 for large lead-acid batteries. The learning rate for residual costs in lead-acid batteries is 20%, a discovery with policy implications. Neglecting to consider cost reductions in lead-acid batteries could result in failure of energy storage start-ups and public policy programs. Generalizing this result, learning in incumbent technologies must be understood to assess the potential of emerging ones.     

Solar-assisted heat pump system and in-ground energy storage in a school building

An experimental solar-assisted heat pump system with a hybrid ground-coupled storage at the F.U.L. in Arlon, Belgium, is described. It includes a 382 m² solar roof, two types of water storages, heat storage in earth by horizontal exchangers, and heat pumps. One operating period (1984–1985) is analyzed. The data processed has shown that each of the subsystems has apparently performed adequately: annual collector efficiency is 0.41, heat pump C.O.P. range around 4. Despite important energy losses from the underground storage, the storage efficiency reaches 0.7. This effectiveness is mainly due to heat recovery below natural soil temperature and also to the use of buried tanks for short-term storage. The main difficulties are controlling the flow between these subsystems and developing an operating strategy that matches both the building's heat requirements and a good solar fraction.     

An optimal design analysis of a novel parabolic trough lighting and thermal system

In order to reduce the cost and improve the efficiency of daylighting, an innovative parabolic trough solar lighting and thermal (PTL/T) system is designed and analyzed in this paper. Parabolic trough solar lighting and thermal system uses parabolic trough collector (PTC) controlled by two‐axis solar tracking system as solar collector. The collected sunlight is split by a cold mirror into visible light and infrared. The visible light is reflected by cold mirror, re‐concentrated by a second‐stage Fresnel lens, and then delivered by plastic optical fiber to the buildings for daylighting. The infrared goes through cold mirror, reaches thermal system, and is used for heating generation. The basic structure of PTL/T was outlined and described. The dimension of fiber bundle and parabolic trough was chosen after an optimal analysis. The cost of illuminating unit area was expressed as a function of illumination space dimensions and critical components efficiency. A case study was conducted to get a specific optimized illumination area and PTC area for the first time. The optimized result is to use 8‐m² PTC as collector to illuminate 500‐m² office space. The total solar energy utilization efficiency is 39.4%, with the lighting efficiency of 16.3% and thermal efficiency of 23.1%. The maximum energy savings and simple payback period were calculated for 10 typical cities when applied in residential, commercial, and industrial sectors. The amounts of greenhouse gas‐emission reductions were also calculated. The payback period in Sunbelt region is as low as less than 10 years like in Los Angeles. The results show the proposed PTL/T system is competitive compared with traditional solar energy systems. Copyright © 2016 John Wiley & Sons, Ltd.     

Application of the genetic algorithm for optimisation of large solar hot water systems

An implementation of the genetic algorithm in a design support tool for (large) solar hot water systems is described. The tool calculates the yield and the costs of solar hot water systems based on technical and financial data of the system components. The genetic algorithm allows for optimisation of separate variables such as the collector type, the number of collectors, the heat storage mass and the collector heat exchanger area. Optimisation can be focussed on, for example, payback time and CO₂ emission reduction. Constraints such as maximum initial costs and installation space are taken into account. The applicability of the genetic algorithm was tested for optimisation of large solar hot water systems. Among others, the sensitivity of the optimum system design to the tap water draw-off and the draw-off pattern has been determined using the optimisation algorithm. As the genetic algorithm is a discrete optimisation tool and is implemented in the design tool through the use of databases, the number of variables in principle is free of choice.     

Numerical simulation of a solar-assisted ejector air conditioning system with cold storage

Indoor space cooling represents a large potential for solar energy use due to the relative coincidence between energy availability and cooling demand. Solar-assisted air conditioning (AC) applications emerged with the development of high efficiency solar collectors. Energy storage (hot or cold) must be implemented for solar-assisted AC applications when cooling demand is present during intervals without available solar energy and also for cooling capacity optimisation (“peak shaving”).The present paper analyses a solar-assisted ejector cooling system with cold storage. Simulations were carried out over one year considering climatic data for a hot location (Béchar, Algeria) and the performance of the system was assessed for a set of design conditions. Effects of cold storage upon comfort conditions and energy demand were evaluated. Maximum room temperature and overall interval of time during which the room temperature exceeded the set-point value were the parameters used to quantify system performance. It was found that cold storage improved comfort conditions compared to a system without storage. For some design conditions it was found that increasing the cold storage capacity did not result in improved comfort conditions. The control algorithm of the system was identified as the cause of this behaviour.     

Embodied energy analysis of photovoltaic (PV) system based on macro- and micro-level

In this paper the energy payback time and CO₂ emissions of photovoltaic (PV) system have been analyzed. The embodied energy for production of PV module based on single crystal silicon, as well as for the manufacturing of other system components have been computed at macro- and micro-level assuming irradiation of 800–1200 W/m² in different climatic zones in India for inclined surface. The energy payback time with and without balance-of-system for open field and rooftop has been evaluated. It is found that the embodied energy at micro-level is significantly higher than embodied energy at macro-level. The effect of insolation, overall efficiency, lifetime of PV system on energy pay back time and CO₂ emissions have been studied with and without balance of system. A 1.2 kW_p PV system of SIEMENS for mudhouse at IIT, Delhi based on macro- and micro-level has been evaluated. The CO₂ mitigation potential, the importance and role of PV system for sustainable development are also highlighted.     

Feasibility analysis of renewable energy supply options for a grid-connected large hotel

This study presents an analysis of the technical and financial viability of grid-only, RES-only and grid/RES hybrid power supply configurations for a large-scale grid-connected hotel (over 100 beds). Assessment criteria comprised net present cost (NPC), renewable fraction (RF) and payback time. The RES software HOMER (National Renewable Energy Laboratory, US) was utilised as the assessment tool with modeling performed with hourly load data input from a hotel located in a subtropical coastal area of Queensland, Australia. The results demonstrate that RES, in principle, has the potential to supply significant power for a large-scale tourist accommodation, in conjunction with the grid-electricity supply. Optimisation modeling demonstrated that, at 2004 prices, the NPC of the grid/RES hybrid configuration is comparable with the grid-only supply and resulted in a RF of 73%, a payback time of 14 years and a reduction in greenhouse gas emissions of 65%. Optimisation modeling also showed that whilst a RES-only configuration can potentially supply 100% of power demand, such a configuration is presently uneconomical given current electricity costs. Results indicate that wind energy conversion systems (WECS), rather than photovoltaics, are the most economically viable RES technology for large-scale grid-connected operations. Specifically, large-scale WECS (over 1000 kW) are more efficient and more economical than multiple small-scale WECS (0.1–100 kW). Hydrogen fuel cells and storage are presently uneconomical in grid-connected configurations. Sensitivity analysis demonstrated that operations that rely on grid-only supply are more economically susceptible to escalations in electricity costs and the imposition of carbon taxes, in comparison to grid/RES hybrids. Indeed, at present electricity prices, which have nearly quadrupled since 2004, the grid/RES hybrid is more economical over a 20-year span than the grid-only system, with a NPC which is 50% lower, and a payback time of 4.5 years. The analysis demonstrates that RES is both technically feasible and economically viable as an addition to grid-connected supply for large-scale tourist operations, and should become more attractive as costs of conventional supplies increase.