Use of parabolic trough solar collectors for solar refrigeration and air-conditioning applications
Introduction
The energy demand associated with air-conditioning in most industrialized countries has been increasing noticeably in recent years, causing peaks in electricity consumption during warm weather periods. This situation is provoked by improved living standards and building occupant comfort demands and architectural characteristics and trends, such as an increasing ratio of transparent to opaque areas in the building envelope [1]. The above, along with refrigeration requirements in the food processing field and the conservation of pharmaceutical products in developing countries, are leading the interest in air-conditioning and refrigeration systems powered by renewable energies, especially solar thermal, which work efficiently, and in certain cases, approach competitiveness with conventional cooling systems.
Solar thermal systems, in addition to the well-known advantages of renewable resources (environmentally-friendly, naturally replenished, distributed,…), are very suitable for air-conditioning and refrigeration demands, because solar radiation availability and cooling requirements usually coincide seasonally and geographically [2], [3]. Solar air-conditioning and refrigeration facilities can also be easily combined with space heating and hot-water applications, increasing the yearly solar fraction of buildings.
In spite of the tremendous research effort made in theoretical analysis and experimental projects since the 70s, and the enormous interest related to solar air-conditioning and refrigeration systems, their commercial implementation is still at a very early stage, due mainly to the high costs associated with these systems and the clear market supremacy of conventional compression chillers. Other obstacles to their large-scale application are the shortage of small power equipment and the lack of practical experience and acquaintance among architects, builders and planners with their design, control and operation [4].
In the Solar Heating and Cooling Technology Roadmap published by International Solar Energy in 2012 [5], it is mentioned a study by Mugnier and Jakob [6] estimating in 750 the number of installed solar cooling systems worldwide in 2011, including small capacity (<20 kW) plants. The IEA roadmap also mentions recent developments of big plants, as that of the United World College in Singapore, completed in 2011, with a cooling capacity of 1470 kW and a collector field of 3900 m2 executed on an energy services company (ESCO) model. Also according to the findings of the study, there is a big potential market for residential applications in Central Europe and in dry and sunny climates areas (Middle East, Australia, Mediterranean islands) although it is constrained by the scarcity of technology options and the difficulties to profit the economy of scale, as well as its dependence on the overall construction sector trends. For example, the case of the Spanish market fall, where the economic crisis has caused a sharp slowdown in growth in the number of projects for residential installations after to lead in 2007 the worldwide market of small size solar cooling systems.
Regarding the type, use and location of the systems, according to the Task 38 of the International Energy Agency survey dated November 2009 [7] the prevailing, 74% for large scale and 90% for small scale (<20 kW), solar cooling installations are those based on thermally driven closed sorption cycles [8], [9] especially LiBr–H 2O single-effect absorption chillers [10] fed by flat-plate and evacuated tube collectors, 46% and 40%, respectively, for large systems. Most of the facilities at 2009 were installed in Europe, having Spain, Germany and Italy more than the 60% of the worldwide solar cooling power at that moment. In regard to the use, air conditioning was the main application because two thirds of small scale systems were devoted to offices and private buildings and the half of the large scale systems were used in offices. The geographical distribution of facilities is expected to be spread to other areas as Asia [11] because the present limitations for projects financing in Europe.
Apart from the studies by the International Energy Agency (IEA) in the Solar Heating and Cooling (SHC) program initiated in Task 25Solar-Assisted Air-Conditioning of Buildings [12], ended in 2004 and followed from 2006 to 2010 by Task 38, Solar Air-Conditioning and Refrigeration [13] and Task 48 (2011–2015) Quality Assurance and Support Measures for Solar Cooling [14], it can be mentioned also UE funded international research, development and technology transfer initiatives as the CLIMASOL project [15] where a complete survey about all the different techniques related to solar air conditioning, useful information and advises as well as in depth description of more than 50 working installations in different countries of the projects partners (Germany, France, Spain, Italy and Greece) were done and continued by the Solar Air Conditioning in Europe SACE project, financed by the European Commission, in which five countries participated analysing about 54 solar air-conditioning facilities [4], [16], SOLAIR (Solar Air-Conditioning) aimed to promote and to strengthen the use of solar air-conditioning systems on residential and commercial applications [17], SOLARCOMBI+ implemented to achieve a better market of small scale solar cooling systems in combination with traditional domestic solar hot water and space heating [18]. HIGH COMBI (High solar fraction heating and cooling system with combination of innovative components and methods) [19] and MEDISCO, MEDiterranean food and agro Industry applications of Solar COoling technologies [20]. In Spain, although experimental and demonstration solar air-conditioning and refrigeration installations are relatively recent [21], apart from the participation of Spanish institutions and companies in the above mentioned projects and programs, there are good references on on-going works in simulation and design tools [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] and experimental evaluation of systems [33], [34], [35], [36], [37], [38], [39], [40] as well as reference national programs as ARFRISOL [41].
The solar cooling systems can be divided [1], [8], [42] into: Closed-cycle thermal systems divided in absorption and adsorption and are rated by their coefficient of performance (COP), which is the ratio of cooling energy produced to thermal energy required; Open-cycle thermal systems based on a combination of air dehumidification by a desiccant that may be liquid or solid, with evaporative cooling; and Mechanical systems use of a solar-powered Rankine-cycle engine connected to a conventional air conditioning system. These can be operated by any type of solar collector, even with PTC [43].
As above mentioned, most of worldwide studies and experiments are based to date on the use of stationary flat-plate collectors (FPC) [29], evacuated tube collectors (ETC), and, to a lesser extent, compound parabolic concentrators (CPC), as the solar heat source in an appropriate operating temperature range (below 100 °C), mainly to feed a single-effect absorption chiller, but also an adsorption chiller or desiccant cooling system [1], [4], [13], [16], [44].
In addition to this, the recent developments in gas-fired double-effect absorption LiBr–H 2O chillers, have made available in the market systems with COP 1.1–1.2 that may high temperature solar-powered sources [45] representing, together to the existing experiences with NH3–H2O absorption chillers, a promising alternative provided investment and maintenance costs of solar concentrators are reasonable. Linear concentrators, either parabolic trough [46], [47], [48], [49], [50] and Fresnel [25], [33], [51], [52], [53] are well suited for this function.
The double-effect absorption cycle permits to take advantage of the higher driving temperature presenting a higher COP, around twice of single-effect cycles [54]. There are two basic configurations for the double-effect cycle depending upon the distribution of the solution through the components, series and parallel flow.
The double-effect absorption cycle (Fig. 1) is characterised by the existence of two desorbers, the high-pressure generator (High-G) and the low-pressure generator (Low-G). PTC collectors provide external heat and vapour is generated in the High-G. The vapour flows to the Low-G, where it changes phase by rejecting heat at sufficiently high temperature that it can be used to separate vapour from the solution flowing through the Low-G. A schematic representation of a parallel flow double-effect LiBr–H 2O absorption cycle is shown in Fig. 1. In the present configuration, the diluted solution leaving the absorber is split into two circuits flowing in parallel. One part of the solution flows to the High-G by passing through the High solution heat exchanger (High-SHX), and the other one goes into the Low-G after passing through the Low solution heat exchanger (Low-SHX). Before entering the absorber, the concentrated solution passes through a Low-SHX where an exchange of sensible heat takes place. The concentrated solution reduces its temperature while the diluted solution gets warmer. The vapour generated in the Low-G is driven directly to the condenser. Finally, the refrigerant separated in the High-G (and condensed in the Low-G) restores its thermodynamic conditions before entering the evaporator. Because optimised use of heat increasing the number of effects.
Section snippets
Parabolic-trough collectors for process heat and solar cooling applications
PTCs are parabolic concentrating systems that focus the direct solar radiation parallel to the collector axis onto a focal line (see Fig. 2). A receiver pipe is installed in this focal line with a heat transfer fluid flowing inside it that absorbs concentrated solar energy from the pipe walls and raises its enthalpy. The collector is provided with a one-axis solar tracking system to ensure that the solar beam falls parallel to its axis.
The tracking mode may be of 1-axis or 2-axis. The solar
Review of cooling systems with PTC
Air-conditioning and refrigeration facilities driven by a PTC solar field are still in frequent despite simulations studies confirms the functional feasibility of this technique in weather favourable regions [95], [96]. Several experimental and test facilities using this technology have appeared in the literature. The first two references in the literature go back to 1957. One of them is a prototype designed and erected at the Laboratoire de l’Energie Solaire in Montlouis (France), where a 1.5-m
Comparative analysis
The advantages of PTCs over the solar collectors traditionally used in solar air-conditioning and refrigeration facilities are their lower thermal losses and, therefore, higher efficiency at the higher working temperature reached, smaller collecting surface for a given power requirement, and no risk of reaching dangerous stagnation temperatures, since in such cases the control system sends the collector into stow position.
The disadvantages of PTCs are that its solar tracking system increases
Results and discussion
Fig. 7 shows a quantitative comparison between the annual efficiency of the different solar collectors considered. For a 1E-chiller the ETC collector is the only with better performance (due to the low operating temperature and better use of diffuse radiation), followed by PTC collectors, this increased performance is not accompanied of higher energy production because the annual energy received Gt, annual by the ETC is lower. For the 2E-chiller PTC collector are more efficient as expected due
Conclusions
In this work, it has been done a comprehensive literature survey on worldwide air-conditioning and refrigeration facilities fed by a PTC solar field. Also, it has been given extensive references on new developments in PTC suitable to be used in process heat and double effect absorption chillers. Findings of this study demonstrate that the yearly rate of grow of this type of installations, excluding 2010 year, is still low, about 4 installations per year especially if compared to the rates of
Nomenclature
- a1
heat loss coefficient [W/(m2 K)]
- a2
temperature dependence of the heat loss coefficient [W/(m2 K2)]
- Abuilding
building area [m2]
- Ac
collector area [m2]
- Aspec
specific collector area [m2/kW]
- b1L, b2L
longitudinal direct incident angle modifier coefficients [-]
- b1T, b2T
transversal direct incident angle modifier coefficients [-]
- C
concentration ratio of collector [-]
- COPrated
rated coefficient of performance [-].
- crf
annual amortization factor [-]
- Eaux
annual auxiliary energy required [kW h]
- Ec
thermal energy given by
Acknowledgements
This work has been supported by the Consejería de Economía, Innovación y Ciencia de la Junta de Andalucía and thanks to the FEDER funds, through the excellence project P10-RNM-5927 “Simulation and Control of parabolic-trough solar thermal collector installations for process heat and cooling applications” as well as by the European Social Fund (ESF) through the Human Potential Operational Programme/POPH.
References (142)
Solar assisted air conditioning of buildings—an overview
Applied Thermal Engineering
(2007)- et al.
Solar thermal air conditioning technology reducing the footprint of solar thermal air conditioning
Renewable and Sustainable Energy Reviews
(2012) - et al.
Solar refrigeration options—a state-of-the-art review
International Journal of Refrigeration
(2008) - et al.
Solar air conditioning in Europe—an overview
Renewable and Sustainable Energy Reviews
(2007) - et al.
Review of solar cooling methods and thermal storage options
Renewable and Sustainable Energy Reviews
(2011) - et al.
A review on solar-powered closed physisorption cooling systems
Renewable and Sustainable Energy Reviews
(2012) - et al.
A review on solar cold production through absorption technology
Renewable and Sustainable Energy Reviews
(2012) - et al.
A review for absorbtion and adsorbtion solar cooling systems in China
Renewable and Sustainable Energy Reviews
(2009) Solar absorption cooling in Spain: perspectives and outcomes from the simulation of recent installations
Renewable Energy
(2006)- et al.
Exergy efficiency analysis in buildings climatized with LiCl–H 2O solar cooling systems that use swimming pools as heat sinks
Energy and Buildings
(2011)
Modelling and simulation of an absorption solar cooling system with low grade heat source in alicante
Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O–LiBr absorption cooling machine
International Journal of Refrigeration
Inverse neural network based control strategy for absorption chillers
Renewable Energy
Design and test results of a low-capacity solar cooling system in Alicante (Spain)
Solar Energy
Energy and economic analysis of an integrated solar absorption cooling and heating system in different building types and climates
Applied Energy
A comparison of solar absorption system configurations
Solar Energy
Optimization of a solar cooling system with interior energy storage
Solar Energy
Model development and validation of a solar cooling plant
International Journal of Refrigeration
Solar absorption cooling plant in Seville
Solar Energy
Experimental results of different control strategies in a solar air-conditioning system at part load
Solar Energy
Experimental evaluation of a direct air-cooled lithium bromide–water absorption prototype for solar air conditioning
Applied Thermal Engineering
Solar space heating and cooling for Spanish housing: potential energy savings and emissions reduction
Solar Energy
Stationary analysis of a solar LiBr–H 2O absorption refrigeration system
International Journal of Refrigeration
Monitoring and simulation of an existing solar powered absorption cooling system in Zaragoza (Spain)
Applied Thermal Engineering
Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building
Renewable Energy
A review of thermally activated cooling technologies for combined cooling, heating and power systems
Progress in Energy and Combustion Science
Study of a new solar adsorption refrigerator powered by a parabolic trough collector
Applied Thermal Engineering
Review of solar sorption refrigeration technologies: development and applications
Renewable and Sustainable Energy Reviews
A review for research and new design options of solar absorption cooling systems
Renewable and Sustainable Energy Reviews
Options for solar-assisted refrigeration-trough collectors and double-effect chillers
Renewable Energy
Parabolic-trough solar collectors and their applications
Renewable and Sustainable Energy Reviews
Concentrated solar energy applications using Fresnel lenses: a review
Renewable and Sustainable Energy Reviews
Investigation of the potential of application of single effect and multiple effect absorption cooling systems
Energy Conversion and Management
Design and construction of a two-axis Sun tracking system for parabolic trough collector (PTC) efficiency improvement
Renewable Energy
A review of principle and sun-tracking methods for maximizing solar systems output
Renewable and Sustainable Energy Reviews
Parabolic trough collectors for industrial process heat in Cyprus
Energy
Direct steam generation in parabolic troughs: final results and conclusions of the DISS project
Energy
Review of R&D progress and practical application of the solar photovoltaic/thermal (PV/T) technologies
Renewable and Sustainable Energy Reviews
High temperature solar heating and cooling systems for different Mediterranean climates: dynamic simulation and economic assessment
Applied Thermal Engineering
Solar sorption cooling systems for residential applications: options and guidelines
International Journal of Refrigeration
The production of cold by means of solar radiation
Solar Energy
Operation and performance of the University of Florida solar air-conditioning system
Solar Energy
Performance analysis and loadmatching for tracking cylindrical parabolic collectors for solar cooling in arid zones
Energy Conversion and Management
Development of a solar-energy-operated vapour-absorption-type refrigerator
Applied Energy
Keeping cool with the Sun
International Sustainable Energy Review
Cited by (160)
Life cycle assessment of a parabolic solar cooker and comparison with conventional cooking appliances
2023, Sustainable Production and ConsumptionSustainability framework of intelligent social houses with a synergy of double-façade architecture and active air conditioning systems
2023, Energy Conversion and Management