Modeling and simulation of solar absorption system performance in Beirut

An analytical study is performed on solar energy utilization in space cooling of a small residential application using a solar lithium bromide absorption system. A simulation program for modeling and performance evaluation of the solar-operated absorption cycle is done for all possible climatic conditions of Beirut. The results have shown that for each ton of refrigeration it is required to have a minimum collector area of 23.3 m² with an optimal water storage tank capacity ranging from 1000 to 1500 liters for the system to operate solely on solar energy for about seven hours a day. The monthly solar fraction of total energy use in cooling is determined as a function of solar collector area and storage tank capacity.An economic assessment is performed based on current cost of conventional cooling systems. It is found that the solar cooling system is marginally competitive only when combined with domestic water heating.     

Comparative study of various optimization criteria for SDHWS and a suggestion for a new global evaluation

This study compares various optimization criteria for a solar domestic hot water system (SDHWS). First of all, we present the various parameters used to evaluate a SDHWS. We consider the energetic, exergetic, environmental (CO₂ emissions) and financial (life cycle cost) analysis. Various optimization criteria of a standard solar hot water system are then proposed. The optimized solutions are compared with a standard hot water system. The most suitable criteria take into account both energetic (therefore environmental) and financial evaluations. The most powerful solutions tend to increase the collector area - increasing the solar fraction during the mid-season - and reduce the tank volume, thereby decreasing the thermal losses and financial cost.Some of the usual evaluation criteria for SDHWs cannot be used as optimization criteria because they do not consider the auxiliary heater, resulting in inaccurate indications of the system’s performance. Therefore, it seemed important to propose a new evaluation method which integrates the life cycle savings, primary energy savings and CO₂ emission savings with regard to a referenced solution based on a radar diagram of these three fractions. This mode of representation is particularly useful when various auxiliary heaters are compared.     

Modelling,simulation and warming impact assessment of a domestic-size absorption solar cooling system

In this paper the modelling, simulation and total equivalent warming impact (TEWI) of a domestic-size absorption solar cooling system is presented. The system consists of a solar collector, storage tank, a boiler and a LiBr–water absorption refrigerator. Experimentally determined heat and mass transfer coefficients were employed in the design and costing of an 11 kW cooling capacity solar driven absorption cooling machine which, from simulations, was found to have sufficient capacity to satisfy the cooling needs of a well insulated domestic dwelling. The system is modelled with the TRNSYS simulation program using appropriate equations predicting the performance of the unit. The final optimum system consists of 15 m² compound parabolic collector tilted at 30° from horizontal and 600 l hot water storage tank. The total life cycle cost of a complete system, comprising the collector and the absorption unit, for a lifetime of 20 years will be of the order of C£ 13,380. The cost of the absorption system alone was determined to be C£ 4800. Economic analysis has shown that for such a system to be economically competitive compared to conventional cooling systems its capital cost should be below C£ 2000. The system however has a lower TEWI being 1.2 times smaller compared to conventional cooling systems.     

Design and analysis of a new solar hydrogen plant for power,methane, ammonia and urea generation

In this paper, a solar power-based combined plant for power, hydrogen, methane, ammonia and urea production is proposed. A parabolic trough collector is utilized for the system prime mover. Moreover, steam Rankine cycle, organic Rankine cycle, hydrogen production and compression subsystem, ammonia, methane and urea production units, single-effect absorption cooling unit, and freshwater production plant are integrated together to develop the present system for better system performance and cost-effectiveness and reduced environmental impact. In order to analyze and evaluate the proposed multigeneration plant, thermodynamic, parametric and economic studies are performed. According to the assessment results, it is found that energetic and exergetic efficiencies of the present multigeneration plant are 66.12% and 61.56%, respectively. The comparisons of the subsystem and overall plant efficiencies show that the highest energetic and energetic efficiencies belong to freshwater production plant by 79.24% and 75.62%, respectively. In addition, the present parametric analysis indicates that an increase in the reference temperature, solar radiation intensity and working pressure of the solar process has a positive effect on the plant's performance. The cost analysis reveals that as the solar radiation intensity and the working pressure of the solar process increase, the hydrogen generation cost decreases. Furthermore, the hydrogen generation cost is achieved to be 1.94 $/kgH₂ at 650 W/m² of the solar radiation intensity, with other parameters remaining constant.     

Modelling and simulation of an absorption solar cooling system for Cyprus

In this paper a modelling and simulation of an absorption solar cooling system is presented. The system is modelled with the TRNSYS simulation program and the typical meteorological year file containing the weather parameters of Nicosia, Cyprus. Initially a system optimisation is carried out in order to select the appropriate type of collector, the optimum size of storage tank, the optimum collector slope and area, and the optimum thermostat setting of the auxiliary boiler. The final optimised system consists of a 15-m² compound parabolic collector tilted 30° from the horizontal and a 600-l hot water storage tank. The collector area is determined by performing the life cycle analysis of the system. The optimum solar system selected gives life cycle savings of C£1376 when a nonsubsidized fuel cost is considered. The system operates with maximum performance when the auxiliary boiler thermostat is set at 87°C. The system long-term integrated performance shows that 84,240 MJ required for cooling and 41,263 MJ for hot water production are supplied with solar energy.     

Performance of a solar-powered air conditioning system in Hong Kong

To study the feasibility of utilizing solar power for comfort cooling in Hong Kong, a solar-powered absorption air-conditioning system was designed and successfully constructed on the campus of the University of Hong Kong (HKU). The system consisted of a flat-plate collector array with a surface area of 38.2 m², a 4.7-kW nominal cooling capacity LiBr---H₂O absorption chiller, a 2.75-m³ hot-water storage tank, a cooling tower, a fan-coil unit, an electrical auxiliary heater, a data-acquisition system and the associated control systems. In the present paper, the design of the HKU solar-powered air-conditioning system is described in detail and its performance over an entire cooling season is also discussed and compared with similar systems in Italy and Singapore. It was found that the HKU solar air-conditioning system had an annual system efficiency of 7.8% and an average solar fraction of 55%.     

Evaluation of adding flash tank to solar combined ejector–absorption refrigeration system

Performance of the absorption cooling system is still a challenge due to the coefficient of performance (COP) that is generally poor when compared with the conventional vapor compression cycle. High solar radiation in hot climates is usually associated with high ambient temperature and consequently peak cooling demand. Absorption cooling cycles can be powered by solar but the performance is limited by heat source temperature (solar collector) and high ambient temperature that can affect the condensation process. Efficiency enhancement of the system components is essential to increase the COP of the system. A modification in the combined absorption–ejector cooling system is adopted. Adding a removable flash tank between the condenser and evaporator could improve entrainment ratio of the ejector, along with improving the cooling effect inside the evaporator. A computer simulation program is developed to evaluate the performance of the modified combined cycle using aqua-ammonia (NH₃–H₂O) refrigerant. The performance of the proposed combined cooling cycle is compared with basic absorption, and combined absorption–ejector cooling cycles. Results showed a significant improvement in the COP of the modified cycle at different operating conditions. Cooling effect and capacity of the evaporator is enhanced due to the reduction of flash gas delivered to the evaporator. Furthermore, the flash tank optimized the ejector entertainment ratio and consequently increasing the condenser pressure. This optimization will enable the system to perform well in hot climates where the condenser efficiency is limited by ambient temperature.     

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.     

Experimental investigation and theoretical analysis of the solar adsorption cooling system in a green building

A solar adsorption cooling system was constructed in the green building of Shanghai Institute of Building Science. The system consisted of evacuated tube solar collector arrays of area 150 m², two adsorption chillers with nominal cooling capacity of 8.5 kW for each and a hot water storage tank of 2.5 m³ in volume. A mathematical model of the system was established. According to experimental results under typical weather condition of Shanghai, the average cooling capacity of the system was 15.3 kW during continuous operation for 8 h. The theoretical analysis of the system was verified and found to agree well with the experimental results. The performance analysis showed that solar radiant intensity had a more distinct influence on the performance of solar adsorption cooling system as compared with ambient temperature. It was observed that the cooling capacity increased with the increase of solar collector area, whereas, solar collecting efficiency varied quite contrary. With the increase of water tank volume, cooling capacity decreased, while, the solar collecting efficiency increased. The system performances can be enhanced by increasing the height-to-diameter ratio of water tank. Additionally, it was observed that solar collecting efficiency decreased with the increase of the initial temperature of water in the tank; however, cooling capacity varied on the contrary. Also can be seen is that optimum nondimensional mass flow rate is 0.7 when the specific mass flow rate exceeds 0.012 kg/m² s.     

Autonomous Adsorption Cooling—Driven by Heat Storage Collected from Solar Heat

This study investigates the performance of an adsorption chiller driven by thermal heat collected from solar collectors’ panels with heat storage. The heat is reserved in a storage tank and the reserved heat is used to drive the adsorption chiller. The investigation was carried on the climatic conditions of Dhaka, Bangladesh. Heat transfer fluid goes from the collectors to the adsorption cooling unit, then from the adsorption cooling unit to the storage tank. It is seen that heat storage is more effective than direct solar coupling; however, it requires more collectors, depending on the size of the storage tank. The analysis shows that cycle time is one of the most influential parameters for the solar-driven adsorption cooling system. It is seen that the size of the collector can be reduced if the proper cycle time is adjusted. The analysis also revealed that the system with 22 collectors (each of 2.415 m²) along with 1000 s cycle time provides better performance for the base run conditions. It is also seen that the solar-driven adsorption chiller with heat storage works well beyond the sunset time.     

Comparative study of internal storage and external storage absorption cooling systems

This work presents a comparative study of the performance of absorption cooling systems with internal storage and with external storage. A full dynamic simulation model including the solar collector field, the absorption heat pump system and the building loads has been performed. The first system is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. The second one is a conventional system composed of one liquid absorption pump and external storage in a water tank. Many batteries of simulations have been done to evaluate the performance of these cooling machines when varying solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. Two different indices have been calculated to analyze the response of both systems: Solar Fraction and Primary Energy Ratio. The comparison between both absorption chillers indicates that in order to reach similar values of storage energy, conventional system has a greater room requirement than four units with internal storage working in parallel, requiring an external water tank of at least 15 m³.     

A computational study on the performance of a solar air-conditioning system with a partitioned storage tank

This paper reports the performance of a modified solar powered air-conditioning system, which is integrated with a partitioned storage tank. In addition, the effect of two main parameters that influence the system performance is presented and discussed. The study shows that by partitioning the storage tank, the solar cooling effect can be realized much earlier and could attain a total solar cooling COP of 12% higher compared to the conventional whole-tank mode. Simulation results also indicate that there exists an optimum ratio of storage tank volume over collector area.     

Solar assisted absorption cooling cycles for reduction of global warming: A multi-objective optimization approach

This work addresses the use of absorption cycles combined with solar energy for reducing the green house gas (GHG) emissions in the cooling sector. The problem of satisfying a given cooling demand at minimum cost and environmental impact is formulated as a bi-criterion non-linear optimization problem that seeks to minimize the total cost of the cooling application and its contribution to global warming. The latter metric, which is assessed following the principles of life cycle assessment (LCA), accounts for the impact caused during the construction and operation of the system. The concept of Pareto optimality is employed to discuss different alternatives for reducing the contribution to global warming that differ in their economic and environmental performance. We also analyze the effect of taxes on CO₂ on the economic and environmental performance of the system. The capabilities of the proposed approach are illustrated through a case study that addresses the design of a solar assisted ammonia-water single effect absorption cooling system with 100 kW of cooling capacity considering Barcelona weather conditions. We show that reducing the contribution to global warming considering the current energy prices and taxes on carbon dioxide emissions is technically viable but economically not appealing. We also discuss the conditions under which reducing the CO₂ emissions could become economically attractive.     

Solar seasonal thermal energy storage for space heating in residential buildings: Optimization and comparison with an air-source heat pump

ABSTRACT

This study evaluates the techno-economics of replacing an air-source heat pump (ASHP) system with a solar seasonal thermal energy storage (STES) system for space heating in Hangzhou, China. Three heating systems, solar STES, ASHP, and ASHP with short-term storage of solar energy, are developed using TRNSYS for a house with 240 m² of floor area. The ratio of tank volume to collector area (RVA) of the STES is optimized for the lowest equivalent annual cost over a lifespan of 20 y. The determined optimal RVA is 0.33 m³/m², although it depends on the system and electricity prices. The optimized STES reduces the electricity demand to 1,269 kWh (74% reduction). Despite the superior energy performance, the economic benefit is only possible with large STES systems, which enjoy low tank prices due to scale effects. The results suggest that policy support is needed for STES, where district scaling is not an option.     

Experimental and theoretical investigation of a solar heating system with heat pump

In this study, the performance of a solar heating system with a heat pump was investigated both experimentally and theoretically. The experimental results were obtained from November to April during the heating season. The experimentally obtained results are used to calculate the heat pump coefficient of performance (COP), seasonal heating performance, the fraction of annual load met by free energy, storage and collector efficiencies and total energy consumption of the systems during the heating season. The average seasonal heating performance values are 4.0 and 3.0 for series and parallel heat pump systems, respectively. A mathematical model was also developed for the analysis of the solar heating system. The model consists of dynamic and heat transfer relations concerning the fundamental components in the system such as solar collector, latent heat thermal energy storage tank, compressor, condenser, evaporator and meteorological data. Some model parameters of the system such as COP, theoretical collector numbers (N_c), collector efficiency, heating capacity, compressor power, and temperatures (T₁, T₂, T₃, T_T) in the storage tank were calculated by using the experimental results. It is concluded that the theoretical model agreed well with the experimental results.     

Yearly simulation of a solar-aided R22-DEGDME absorption heat pump system

The performance of a solar-aided R22-DEGDME absorption heat pump system designed for 100 kW cooling capacity is investigated by a computer simulation using hourly data for Ankara. In summer the generator, and in winter the evaporator, receives solar energy while the remaining demands are met by auxiliary heaters. When needed, these boost the temperature of the water from the storage tank to the minimum allowable levels which are determined as 20°C in winter and over 80°C in summer. The system performance, judged by the fraction of the load supplied from solar energy, is affected mostly from the climate, source temperature limit, collector type and area but little from storage tank size, for the sizes and configuration under investigation. With 400 m² of high efficiency collectors, the solar energy supplied 38% of the demand in winter and 91% of the demand in summer.     

Modeling and optimization of a solar driven membrane distillation desalination system

The desalination technology using membrane distillation driven by solar energy is a feasible solution for reducing the energy cost. A dynamic simulation model for a solar driven membrane distillation desalination system (SMDDS) is developed on the Aspen Custom Modeler^® (ACM) platform for the system performance and optimization study. The rigorous model for the spiral-wound air gap membrane distillation (SP-AGMD) module takes into account the heat and mass transfer resistances associated with each composing layer. The effects of adopting different objective functions, solar radiation conditions, thermal storage tank configurations, as well as the flowrates of the membrane distillation module and the thermal storage tank on the optimized performance are reported. Simple thermal storage tank and lower flowrate of the membrane distillation module are advantageous to higher water production rate. A control system using conventional PI (Proportional/Integral) controllers is proposed and the water production rate can reach about 87% of the optimal result for clear sky operation.     

Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store

A domestic-scale prototype experimental solar cooling system has been developed based on a LiBr/H₂O absorption system and tested during the 2007 summer and autumn months in Cardiff University, UK. The system consisted of a 12 m² vacuum tube solar collector, a 4.5 kW LiBr/H₂O absorption chiller, a 1000 l cold storage tank and a 6 kW fan coil. The system performance, as well as the performances of the individual components in the system, were evaluated based on the physical measurements of the daily solar radiation, ambient temperature, inlet and outlet fluid temperatures, mass flow rates and electrical consumption by component. The average coefficient of thermal performance (COP) of the system was 0.58, based on the thermal cooling power output per unit of available thermal solar energy from the 12 m² Thermomax DF100 vacuum tube collector on a hot sunny day with average peak insolation of 800 W/m² (between 11 and 13.30 h) and ambient temperature of 24 °C. The system produced an electrical COP of 3.6. Experimental results prove the feasibility of the new concept of cold store at this scale, with chilled water temperatures as low as 7.4 °C, demonstrating its potential use in cooling domestic scale buildings.     

A comparison of solar absorption system configurations

Solar thermal driven cooling systems for residential applications are a promising alternative to electric compression chillers, although its market introduction still represents a challenge, mainly due to the higher investment costs. The most common system configuration is an absorption chiller driven by a solar thermal system, backed up by a secondary heating source, normally a gas boiler. Heat storage in the primary (solar) circuit is mandatory to stabilize and extend the operation of the chiller, whereas a cold storage tank is not so common.This paper deals with the selection of the most suitable configuration for residential cooling systems with solar energy. In Spain, where cooling needs are usually higher than heating needs, the interest of a reversible heat pump as auxiliary system and a secondary cooling storage are analyzed.A complete TRNSYS model has been developed to compare a configuration with just hot storage (of typical capacity 40 L/m² of solar collector surface) and a configuration with both, hot and cool storages. The most suitable configuration is very sensible to the solar collector area. As the collector area increases, the advantages of a cool storage vanish. Increasing the collector area tends to increase the temperature of the hot storage, leading to higher thermal losses in both the collector and the tank. When the storage volume is concentrated in one tank, these effects are mitigated. The effect of other variables on the optimal configuration are also analyzed: collector efficiency curve, COP of the absorption chiller, storage size, and temperature set-points of the chillers.     

Parametric Analysis of Solar Heating and Cooling Systems for Residential Applications

Abstract

In this paper, a parametric analysis of two solar heating and cooling systems, one using an absorption heat pump and the other one using an adsorption heat pump, was performed. The systems under investigation were designed to satisfy the energy requirements of a residential building for space heating/cooling purposes and domestic hot water production. The system with the absorption heat pump was analyzed upon varying (i) the solar collectors’ area, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The system with the adsorption heat pump was evaluated upon varying (i) the inlet temperature of hot water supplied to the adsorption heat pump, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The analyses were performed using the dynamic simulation software TRNSYS in terms of primary energy consumption, global carbon dioxide equivalent emissions, and operating costs. The performance of the solar heating and cooling systems was compared with those associated with a conventional system from energy, environmental and economic points of views in order to evaluate the potential benefits.