Performance analysis of hybrid liquid desiccant solar cooling system

This thesis investigates the coefficient of performance (COP) of a hybrid liquid desiccant solar cooling system. This hybrid cooling system includes three sections: 1) conventional air-conditioning section; 2) liquid desiccant dehumidification section and 3) air mixture section. The air handling unit (AHU) with mixture variable air volume design is included in the hybrid cooling system to control humidity. In the combined system, the air is first dehumidified in the dehumidifier and then mixed with ambient air by AHU before entering the evaporator. Experiments using lithium chloride as the liquid desiccant have been carried out for the performance evaluation of the dehumidifier and regenerator. Based on the air mixture (AHU) design, the electrical coefficient of performance (ECOP), thermal coefficient of performance (TCOP) and whole system coefficient of performance (COPsys) models used in the hybrid liquid desiccant solar cooing system were developed to evaluate this system performance. These mathematical models can be used to describe the coefficient of performance trend under different ambient conditions, while also providing a convenient comparison with conventional air conditioning systems. These models provide good explanations about the relationship between the performance predictions of models and ambient air parameters. The simulation results have revealed the coefficient of performance in hybrid liquid desiccant solar cooling systems substantially depends on ambient air and dehumidifier parameters. Also, the liquid desiccant experiments prove that the latent component of the total cooling load requirements can be easily fulfilled by using the liquid desiccant dehumidifier. While cooling requirements can be met, the liquid desiccant system is however still subject to the hysteresis problems.

Optimizing solar collector tilt angle to improve energy harvesting in a solar cooling system

Solar cooling systems are gaining popularity due to continuously increasing of energy costs around the world. However, there are still some factors that are hindering the installation of solar cooling systems on a larger scale. One being the cost associated with the solar collectors required to provide heat to the absorption chiller. This study demonstrates the possibility of reducing the number of solar panels in a residential solar cooling system based on evacuated tubes producing hot water at a low temperature (90 °C) and a water-ammonia absorption chiller.

Low temperature solar cooling system with absorption chiller and desiccant wheel

Nowadays Solar Cooling systems are becoming popular to reduce the carbon footprint of air conditioning. The use of an absorption chiller connected to solar thermal panels is increasing, but little study has been carried out to assess the advantage of join together an absorption chiller and a desiccant wheel to remove the sensible heat and the latent heat in different ways than the current design adopted in the industry. In this work I assess the possibility of implement a desiccant wheel in a conventional solar cooling system and the possibility of recovering the heat rejected by the absorption chiller which is then used for the regeneration of the desiccant wheel. The implementation of a desiccant wheel and the recovery of the heat rejected could provide a significant energy saving when compared to traditional solar cooling system. The results assist in the practical development of a solar cooling system which simultaneously uses absorption and adsorption technology.

System Integration of PV/T Collectors in Solar Cooling Systems

The demand for cooling and air-conditioning of building is increasingly ever growing. This increase is mostly due to population and economic growth in developing countries, and also desire for a higher quality of thermal comfort. Increase in the use of conventional cooling systems results in larger carbon footprint and more greenhouse gases considering their higher electricity consumption, and it occasionally creates peaks in electricity demand from power supply grid. Solar energy as a renewable energy source is an alternative to drive the cooling machines since the cooling load is generally high when solar radiation is high. This thesis examines the performance of PV/T solar collector manufactured by Solarus company in a solar cooling system for an office building in Dubai, New Delhi, Los Angeles and Cape Town. The study is carried out by analyzing climate data and the requirements for thermal comfort in office buildings. Cooling systems strongly depend on weather conditions and local climate. Cooling load of buildings depend on many parameters such as ambient temperature, indoor comfort temperature, solar gain to the building and internal gains including; number of occupant and electrical devices. The simulations were carried out by selecting a suitable thermally driven chiller and modeling it with PV/T solar collector in Polysun software. Fractional primary energy saving and solar fraction were introduced as key figures of the project to evaluate the performance of cooling system. Several parametric studies and simulations were determined according to PV/T aperture area and hot water storage tank volume. The fractional primary energy saving analysis revealed that thermally driven chillers, particularly adsorption chillers are not suitable to be utilizing in small size of solar cooling systems in hot and tropic climates such as Dubai and New Delhi. Adsorption chillers require more thermal energy to meet the cooling load in hot and dry climates. The adsorption chillers operate in their full capacity and in higher coefficient of performance when they run in a moderate climate since they can properly reject the exhaust heat. The simulation results also indicated that PV/T solar collector have higher efficiency in warmer climates, however it requires a larger size of PV/T collectors to supply the thermally driven chillers for providing cooling in hot climates. Therefore using an electrical chiller as backup gives much better results in terms of primary energy savings, since PV/T electrical production also can be used for backup electrical chiller in a net metering mechanism.

Analisi energetica e studio di fattibilità di un impianto di solar cooling a servizio di un hotel

Il calore del sole diventa energia frigorifera sostenibile economicamente ed energticamente. Gli sviluppi tecnologici degli ultimi anni nel campo dell’energia solare consentono consentono di realizzare quanto apparentemente in contraddizione con i fondamentali teoremi della fisica classica ed in particolare della termodinamica. Questo elaborato vuole analizzare come, se e in quali casi sia conveniente l'installazione di un impianto di solar cooling per la climatizzazione estiva di un edificio. Partendo dal problema, sempre più attuale, dei crescenti consumi di energia elettrica per il condizionamento estivo degli edifici, dalla ricerca bibliografica è emerso come inizi a farsi strada, come soluzione percorribile e logicamente interessante, l’utilizzo dell’energia solare, fruttata in impianti che prendono il nome di impianti di solar cooling.

Demonstration of Solar Heating and Cooling System using Sorption Integrated Solar Thermal Collectors

Producing cost-competitive small and medium-sized solar cooling systems is currently a significant challenge. Due to system complexity, extensive engineering, design and equipment costs; the installation costs of solar thermal cooling systems are prohibitively high. In efforts to overcome these limitations, a novel sorption heat pump module has been developed and directly integrated into a solar thermal collector. The module comprises a fully encapsulated sorption tube containing hygroscopic salt sorbent and water as a refrigerant, sealed under vacuum with no moving parts. A 5.6m2 aperture area outdoor laboratory-scale system of sorption module integrated solar collectors was installed in Stockholm, Sweden and evaluated under constant re-cooling and chilled fluid return temperatures in order to assess collector performance. Measured average solar cooling COP was 0.19 with average cooling powers between 120 and 200 Wm-2 collector aperture area. It was observed that average collector cooling power is constant at daily insolation levels above 3.6 kWhm-2 with the cooling energy produced being proportional to solar insolation. For full evaluation of an integrated sorption collector solar heating and cooling system, under the umbrella of a European Union project for technological innovation, a 180 m2 large-scale demonstration system has been installed in Karlstad, Sweden. Results from the installation commissioned in summer 2014 with non-optimised control strategies showed average electrical COP of 10.6 and average cooling powers between 140 and 250 Wm-2 collector aperture area. Optimisation of control strategies, heat transfer fluid flows through the collectors and electrical COP will be carried out in autumn 2014.

Evaluation of a thermally driven heat pump for solar heating and cooling applications

Exploiting solar energy technology for both heating and cooling purposes has the potential of meeting an appreciable portion of the energy demand in buildings throughout the year. By developing an integrated, multi-purpose solar energy system, that can operate all twelve months of the year, a high utilisation factor can be achieved which translates to more economical systems. However, there are still some techno-economic barriers to the general commercialisation and market penetration of such technologies. These are associated with high system and installation costs, significant system complexity, and lack of knowledge of system implementation and expected performance. A sorption heat pump module that can be integrated directly into a solar thermal collector has thus been developed in order to tackle the aforementioned market barriers. This has been designed for the development of cost-effective pre-engineered solar energy system kits that can provide both heating and cooling. This thesis summarises the characterisation studies of the operation of individual sorption modules, sorption module integrated solar collectors and a full solar heating and cooling system employing sorption module integrated collectors. Key performance indicators for the individual sorption modules showed cooling delivery for 6 hours at an average power of 40 W and a temperature lift of 21°C. Upon integration of the sorption modules into a solar collector, measured solar radiation energy to cooling energy conversion efficiencies (solar cooling COP) were between 0.10 and 0.25 with average cooling powers between 90 and 200 W/m2 collector aperture area. Further investigations of the sorption module integrated collectors implementation in a full solar heating and cooling system yielded electrical cooling COP ranging from 1.7 to 12.6 with an average of 10.6 for the test period. Additionally, simulations were performed to determine system energy and cost saving potential for various system sizes over a full year of operation for a 140 m2 single-family dwelling located in Madrid, Spain. Simulations yielded an annual solar fraction of 42% and potential cost savings of €386 per annum for a solar heating and cooling installation employing 20m2 of sorption integrated collectors.

Integrated solar energy and absorption cooling model for HVAC (heating, ventilating, and air conditioning) applications in buildings

The demands in production and associate costs at power generation through non renewable resources are increasing at an alarming rate. Solar energy is one of the renewable resource that has the potential to minimize this increase. Utilization of solar energy have been concentrated mainly on heating application. The use of solar energy in cooling systems in building would benefit greatly achieving the goal of non-renewable energy minimization. The approaches of solar energy heating system research done by initiation such as University of Wisconsin at Madison and building heat flow model research conducted by Oklahoma State University can be used to develop and optimize solar cooling building system. The research uses two approaches to develop a Graphical User Interface (GUI) software for an integrated solar absorption cooling building model, which is capable of simulating and optimizing the absorption cooling system using solar energy as the main energy source to drive the cycle. The software was then put through a number of litmus test to verify its integrity. The litmus test was conducted on various building cooling system data sets of similar applications around the world. The output obtained from the software developed were identical with established experimental results from the data sets used. Software developed by other research are catered for advanced users. The software developed by this research is not only reliable in its code integrity but also through its integrated approach which is catered for new entry users. Hence, this dissertation aims to correctly model a complete building with the absorption cooling system in appropriate climate as a cost effective alternative to conventional vapor compression system.

Heat reject recovery in solar air conditioning

This study demonstrates the possibility of using an absorption chiller to produce chilled water for air conditioning, and at the same time recover the rejected heat producing domestic hot water. The absorption chiller considered for this application has been sized to suit a standard household and uses a solution of ammonia and water running on hot water at a temperature ranging from 80 - 120°C produced by thermal solar panels. The system consists of five main components: generator, rectifier, condenser, evaporator and absorber, and is divided in two sections at two different pressures. The section at higher pressure includes the generator, rectifier and condenser whereas the section at lower pressure includes the evaporator and the absorber. Heat in this type of system is usually rejected to the environment from the condenser, rectifier and absorber through a cooling tower or air cooler exchanger. In this paper we describe how to recover this heat to create domestic hot water by providing a quantitative evaluation of the amount of energy recovered by the proposed system, if used in the Australian region.

TCA Evaluation : Lab Measurements, Modelling and System Simulations

The study reported here is part of a large project for evaluation of the Thermo-Chemical Accumulator (TCA), a technology under development by the Swedish company ClimateWell AB. The studies concentrate on the use of the technology for comfort cooling. This report concentrates on measurements in the laboratory, modelling and system simulation. The TCA is a three-phase absorption heat pump that stores energy in the form of crystallised salt, in this case Lithium Chloride (LiCl) with water being the other substance. The process requires vacuum conditions as with standard absorption chillers using LiBr/water. Measurements were carried out in the laboratories at the Solar Energy Research Center SERC, at Högskolan Dalarna as well as at ClimateWell AB. The measurements at SERC were performed on a prototype version 7:1 and showed that this prototype had several problems resulting in poor and unreliable performance. The main results were that: there was significant corrosion leading to non-condensable gases that in turn caused very poor performance; unwanted crystallisation caused blockages as well as inconsistent behaviour; poor wetting of the heat exchangers resulted in relatively high temperature drops there. A measured thermal COP for cooling of 0.46 was found, which is significantly lower than the theoretical value. These findings resulted in a thorough redesign for the new prototype, called ClimateWell 10 (CW10), which was tested briefly by the authors at ClimateWell. The data collected here was not large, but enough to show that the machine worked consistently with no noticeable vacuum problems. It was also sufficient for identifying the main parameters in a simulation model developed for the TRNSYS simulation environment, but not enough to verify the model properly. This model was shown to be able to simulate the dynamic as well as static performance of the CW10, and was then used in a series of system simulations. A single system model was developed as the basis of the system simulations, consisting of a CW10 machine, 30 m2 flat plate solar collectors with backup boiler and an office with a design cooling load in Stockholm of 50 W/m2, resulting in a 7.5 kW design load for the 150 m2 floor area. Two base cases were defined based on this: one for Stockholm using a dry cooler with design cooling rate of 30 kW; one for Madrid with a cooling tower with design cooling rate of 34 kW. A number of parametric studies were performed based on these two base cases. These showed that the temperature lift is a limiting factor for cooling for higher ambient temperatures and for charging with fixed temperature source such as district heating. The simulated evacuated tube collector performs only marginally better than a good flat plate collector if considering the gross area, the margin being greater for larger solar fractions. For 30 m2 collector a solar faction of 49% and 67% were achieved for the Stockholm and Madrid base cases respectively. The average annual efficiency of the collector in Stockholm (12%) was much lower than that in Madrid (19%). The thermal COP was simulated to be approximately 0.70, but has not been possible to verify with measured data. The annual electrical COP was shown to be very dependent on the cooling load as a large proportion of electrical use is for components that are permanently on. For the cooling loads studied, the annual electrical COP ranged from 2.2 for a 2000 kWh cooling load to 18.0 for a 21000 kWh cooling load. There is however a potential to reduce the electricity consumption in the machine, which would improve these figures significantly. It was shown that a cooling tower is necessary for the Madrid climate, whereas a dry cooler is sufficient for Stockholm although a cooling tower does improve performance. The simulation study was very shallow and has shown a number of areas that are important to study in more depth. One such area is advanced control strategy, which is necessary to mitigate the weakness of the technology (low temperature lift for cooling) and to optimally use its strength (storage).

Performance analysis of solar desiccant-evaporative cooling for a commercial building under different Australian climates

This paper evaluates and compares the system performance of a solar desiccant-evaporative cooling (SDEC) system with a referenced conventional variable air volume (VAV) system for a typical office building in all 8 Australian capital cities. A simulation model of the building is developed using the whole building simulation software EnergyPlus. The performance indicators for the comparison are system coefficient of performance (COP), annual primary energy consumption, annual energy savings, and annual CO2 emissions reduction. The simulation results show that Darwin has the most apparent advantages for SDEC system applications with an annual energy savings of 557 GJ and CO2 emission reduction of 121 tonnes. The maximum system COP is 7. For other climate zones such as Canberra, Hobart and Melbourne, the SDEC system is not as energy efficient as the conventional VAV system.

Adsorption study of silica gel particle for improvement in design of adsorption beds used in solar driven cooling units

Simulations using Ansys Fluent 6.3.26 have been performed to look into the adsorption characteristics of a single silica gel particle exposed to saturated humid air streams at Re=108 & 216 and temperature of 300K. The adsorption of the particle has been modeled as a source term in the species and the energy equations using a Linear Driving Force (LDF) equation. The interdependence of the thermal and the water vapor concentration field has been analysed. This work is intended to aid in understanding the adsorption effects in silica gel beds and in their efficient design. (C) 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Feasibility Study of Solar Driven Underground Cooling System

In the United States the peak electrical use occurs during the summer. In addition, the building sector consumes a major portion of the annual electrical energy consumption. One of the main energy consuming components in the building sector is the Heating, Ventilation, and Air-Conditioning (HVAC) systems. This research studies the feasibility of implementing a solar driven underground cooling system that could contribute to reducing building cooling loads. The developed system consists of an Earth-to-Air Heat Exchanger (EAHE) coupled with a solar chimney that provides a natural cool draft to the test facility building at the Solar Energy Research Test Facility in Omaha, Nebraska. Two sets of tests have been conducted: a natural passively driven airflow test and a forced fan assisted airflow test. The resulting data of the tests has been analyzed to study the thermal performance of the implemented system. Results show that: The underground soil proved to be a good heat sink at a depth of 9.5ft, where its temperature fluctuates yearly in the range of (46.5°F-58.2°F). Furthermore, the coupled system during the natural airflow modes can provide good thermal comfort conditions that comply with ASHRAE standard 55-2004. It provided 0.63 tons of cooling, which almost covered the building design cooling load (0.8 tons, extreme condition). On the other hand, although the coupled system during the forced airflow mode could not comply with ASHRAE standard 55-2004, it provided 1.27 tons of cooling which is even more than the building load requirements. Moreover, the underground soil experienced thermal saturation during the forced airflow mode due to the oversized fan, which extracted much more airflow than the EAHE ability for heat dissipation and the underground soil for heat absorption. In conclusion, the coupled system proved to be a feasible cooling system, which could be further improved with a few design recommendations.

Modern solar maximum forced late twentieth century Greenland cooling

The abrupt Northern Hemispheric warming at the end of the twentieth century has been attributed to an enhanced greenhouse effect. Yet Greenland and surrounding subpolar North Atlantic remained anomalously cold in 1970s to early 1990s. Here we reconstructed robust Greenland temperature records (North Greenland Ice Core Project and Greenland Ice Sheet Project 2) over the past 2100 years using argon and nitrogen isotopes in air trapped within ice cores and show that this cold anomaly was part of a recursive pattern of antiphase Greenland temperature responses to solar variability with a possible multidecadal lag. We hypothesize that high solar activity during the modern solar maximum (approximately 1950s–1980s) resulted in a cooling over Greenland and surrounding subpolar North Atlantic through the slowdown of Atlantic Meridional Overturning Circulation with atmospheric feedback processes.

Dry cooling with night cool storage to enhance solar power plants performance in extreme conditions areas

Solar thermal power plants are usually installed in locations with high yearly average solar radiation, often deserts. In such conditions, cooling water required for thermodynamic cycles is rarely available. Moreover, when solar radiation is high, ambient temperature is very high as well; this leads to excessive condensation temperature, especially when air-condensers are used, and decreases the plant efficiency. However, temperature variation in deserts is often very high, which drives to relatively low temperatures during the night. This fact can be exploited with the use of a closed cooling system, so that the coolant (water) is chilled during the night and store. Chilled water is then used during peak temperature hours to cool the condenser (dry cooling), thus enhancing power output and efficiency. The present work analyzes the performance improvement achieved by night thermal cool storage, compared to its equivalent air cooled power plant. Dry cooling is proved to be energy-effective for moderately high day–night temperature differences (20 °C), often found in desert locations. The storage volume requirement for different power plant efficiencies has also been studied, resulting on an asymptotic tendency.