Use of parabolic trough solar collectors for solar refrigeration and air-conditioning applications

Abstract

The increasing energy demand for air-conditioning in most industrialized countries, as well as refrigeration requirements in the food processing field and the conservation of pharmaceutical products, is leading to a growing interest in solar cooling systems. So far, the more commonly systems used are single-effect water/lithium bromide absorption chillers powered by flat-plate or evacuated tube collectors operating with COP of about 0.5–0.8 and driving temperatures of 75–95  °C. In general terms, performance of thermally driven cooling systems increases to about 1.1–1.4 using double-effect cycles fed by higher temperature sources (140–180 °C). If solar energy is to be used, concentrating technologies must be considered. Although some experiences on the integration of parabolic trough collectors (PTC) and Fresnel lenses in cooling installations can be found in the literature, the quantity is far to be comparable to that of low temperature collectors. Some manufacturers have undertaken the development of modular, small, lightweight and low cost parabolic collectors, compatible for installation on the roofs of the buildings aiming to overcome some of the current technology drawbacks as costs and modularity. After a comprehensive literature review, this work summarises the existing experiences and realizations on applications of PTC in solar cooling systems as well as present a survey of the new collectors with potential application in feeding double effect absorption chillers. In addition to this, it is evaluated its use as an occasional alternative to other solar thermal collectors in air conditioning applications by dynamical simulation. Results for the case studies developed in this work show that PTC present similar levelized costs of energy for cooling than flat plate collector (FPC) and lower than evacuated tube collectors (ETC) and compound parabolic collectors (CPC).

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.