Solar Cooling

The Earthscan Expert Guide to Solar Cooling Systems

Paul Kohlenbach, Uli Jakob

• Publisher: Routledge
• Year: 2014
• Print ISBN: 9780367787417
• eBook ISBN: 9781317963974
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Ecology

Cooling buildings is a major global energy consumer and the energy requirement is growing year by year. This guide to solar cooling technology explains all you need to know about how solar energy can be converted into cooling energy. It outlines the difference between heat-driven and photovoltaic-driven systems and gives examples of both, making clear in what situations solar cooling technology makes sense. It includes chapters on:

• solar thermal collectors

• solar cooling technologies

• cold distribution

• storage components

• designing and sizing

• installation, operation and maintenance

• economic feasibility

• potential markets

• case studies.

Solar Cooling is for engineers, architects, consultancies, solar thermal technology companies, students and anyone who is interested in getting involved with this technology.

1

Introduction

 

 

This book aims to give an overview on the technological and economical aspects of solar cooling systems for buildings, concentrating on larger solar thermal cooling systems. The title term ‘solar cooling’ refers to two categories of systems:

1. Systems providing air-conditioning, e.g. for buildings, usually in the form of either chilled water or air.

2. Systems providing cold, e.g. for industrial processes, usually in the form of chilled water.

The book’s focus is on the provision of air-conditioning for buildings, but systems providing cold for industrial processes are also discussed.

1.1 Active and passive cooling systems

There are two kinds of cooling systems: active and passive. Active systems are classified as those where energy, typically electricity and heat, is used to drive a thermal conversion process that provides some form of cold. Passive systems are systems without any external energy input. Large solar-powered active systems – systems providing cooling for entire buildings, not small solarassisted units for single rooms – are the primary focus of this book, though passive systems will also be briefly discussed.

Worldwide, the energy consumption required for active cooling and air-conditioning is rising rapidly. Standard electrically driven compressor chillers and air-conditioners (typically reverse-cycle split units) have seen a continuous global increase in sales. In particular, the sales figures for split units with a cooling capacity range up to 5 kWr have risen rapidly in recent years. The Japan Refrigeration and Air Conditioning Industry Association (JRAIA) has estimated worldwide sales of these systems to be 93.8 million units in 2012 [1]. To put this in perspective, the number of cars sold worldwide in 2012 was 81 million units [4].

Air-conditioners and chillers draw their maximum energy consumption during their peak-load periods during the summer. In the last few years this has regularly led to grids working to maximum capacity and blackouts in, for example, southern Europe and other regions of the world. Conventional small air-conditioners (also known as split units) have high energy consumption and use non-environmentally friendly refrigerants. The refrigerants currently used in split units no longer have an ozone depletion potential (ODP), but they have a considerable global warming potential (GWP) because of refrigerant leakages in the range of 5–15 per cent per year.

Passive systems – systems without any external energy input – include, for example, solar shading devices, semi-transparent glass covers or solardriven ventilation effects. Passive cooling depends on building construction and thus needs to be considered when designing a new building. One passive cooling option is to design a heavy-walled building (e.g. with concrete walls and ceilings), which reduces the influence of solar radiation and internal loads on the cooling requirement. The heat-storage capacity of the building material provides thermal insulation which keeps heat out of the building. A second option that can be used in new light-walled buildings are phase-change materials (PCMs) which can be integrated into wall plaster or suspended ceilings. The building heat will then be taken up in the PCM during the day, thus cooling the building, and be discharged to the building during night-time to be removed by night ventilation. The PCM option can be applied to new and existing buildings without major rebuilding. Other options are passive cooling systems that prevent external heat input into the building. These include, for example, solar shading devices, semi-transparent glass covers and natural ventilation effects. Passive cooling measures reduce the building’s cooling load. The remaining cooling load is now low enough to be more efficiently covered using a smaller active cooling system. In other words, every unit of heat that is prevented from entering the building through passive cooling measures will save electricity used by the active cooling system.

1.2 Solar cooling

Active solar cooling systems are an attractive alternative to conventional electrically driven air-conditioning. They can be used in virtually all applications where conventional air-conditioning or cooling is used. There are two ways to harness the solar energy to drive active solar cooling systems:

1.solar thermal collectors, converting sun rays into heat; or

2. solar photovoltaic panels, converting sun rays into electrical power.

The difference between the two above possibilities is quite distinct. This book discusses both options in Chapter 3. However, the focus of this book is on solar thermal systems (option 1) as they currently have a higher market share. These combine thermally driven sorption chillers with solar thermal collectors. Sorption chillers use environmentally friendly refrigerants (water or ammonia) and have only very low electricity demand. Therefore the operating costs of these chillers are very low and the CO₂ balance compared to conventional systems is considerably better. In the case of active solar cooling the main advantage is the coincidence of solar irradiation and cooling demand, which matches very well in sunny and hot climates all over the world.

At the time of writing about 1,000 large solar cooling systems are installed worldwide and the market has grown in the last eight years between 40 and 70 per cent per year. The overall number of systems installed to date indicates that solar cooling is still a niche market, but one which is developing. Since 2007, a system cost reduction of about 50 per cent was realized because of further standardization of pre-grouped components, so-called solar cooling kits. Solar cooling is especially appealing if the solar thermal system is also used for other applications such as heating, hot water, etc. Maximum solar system operation time and a low-cost source of driving heat for sorption chillers are key factors in making solar thermal cooling systems economically feasible.

1.2.1 Pre-conditions for solar cooling

All solar cooling systems require three main boundary conditions in order to work well. First, a good solar resource needs to be available – the more sun can be utilised the better. This depends on the geographic location of the building. Second, the climate zone needs to be taken into consideration. Seasonal and daily variations of local climate determine the heating and cooling loads of a building. Third, energy costs should be considered – electricity and fossil fuel costs determine the economics of a solar cooling system. A fourth, less important, condition is the simultaneity of solar radiation and cooling requirement of a building. In other words, the building should ideally have its heating/cooling loads when the sun is shining during the day, and not early in the morning or at night. These boundary conditions are discussed in more detail in this book.

1.2.2 A brief history of solar cooling

The world’s first solar cooling system was operated in Paris, France, during the 1878 world exhibition [2]. The system consisted of an ammonia–water ABsorption chiller and a parabolic reflector, and produced ice blocks. It was purely a demonstration system. The first commercial solar cooling systems for air-conditioning were developed in Europe and the USA 100 years later, in the 1970s, by companies such as Dornier-Prinz Solartechnik (Germany), Arkla Industries (USA, today Robur/Italy) and Carrier (USA). Several demonstration projects were built, motivated by high fossil fuel cost during the global oil crisis of the late 1970s. After oil prices decreased again, the lack of demand on the market for solar cooling resulted in the production of solar cooling systems ceasing in the late 1980s [3].

In the 1990s companies like Yazaki (Japan) and Thermolux (Germany) manufactured and installed a few custom-made solar cooling systems. After 2000, the first solar cooling system suppliers started offering solar cooling systems. Contrary to earlier attempts, this was done with a focus on the whole system, not just the individual components. Typically, these were solar thermal collector suppliers looking to increase their sales volume by including a cooling option with their products. Suppliers included Citrin Solar, Conergy and SolarNext (all from Germany) and SOLID and Sol-ution (both from Austria), among others. The demand led to a number of manufacturers of small-scale ABsorption or ADsorption chillers entering the market. Today, companies like SorTech AG, InvenSor GmbH, EAW (all from Germany), Pink (Austria), SolabCool (Netherlands), Yazaki (Japan) and Thermax (India) provide small capacity chillers with a cooling capacity between 5 kWr and 35 kWr. As we write, the solar cooling market is far from being established; however, rising energy costs and increasing public awareness of environmental issues are likely to change this.

2

Solar thermal collectors

In this chapter we discuss the workings and general composition of solar thermal collectors. The chapter covers all solar thermal collectors suitable for solar cooling systems. The physics behind the technology is also discussed, especially the collector efficiencies.

Essential terms for solar thermal collectors

Absorptivity – the fraction of radiation absorbed at a specific wavelength

Absorber area – the area of a solar thermal collector where radiation energy is converted into thermal energy

Aperture area – the area of a solar thermal collector that collects the sun’s radiation energy (typically larger than the absorber area)

Concentration ratio – the ratio of aperture to absorber area of a concentrating solar collector

Diffuse radiation – solar radiation that interacts with the atmosphere before reaching the Earth’s surface. Diffuse radiation is non-directional. It is caused by scattering of light at molecules in the atmosphere, refection and absorption/re-emission

Direct radiation – solar radiation that does not interact with the atmosphere before reaching the Earth’s surface. Direct radiation is directional and comes straight from the sun

DNI – direct normal irradiation; the direct radiation falling orthogonally onto a collector surface

Emissivity – the relative ability of a material surface to emit energy by radiation. It is expressed as the ratio of energy radiated by a material at a given temperature to energy radiated by a black body at the same temperature

Gross area – the total area a solar thermal collector requires for complete installation on a roof or the ground. Gross area includes space required for access and...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of figures
• List of tables
• Acknowledgements
• 1 Introduction
• 2 Solar thermal collectors
• 3 Solar cooling technologies
• 4 Cold distribution
• 5 Storage in solar thermal cooling systems
• 6 Designing and sizing solar cooling systems
• 7 Installation, operation and maintenance
• 8 Economic feasibility
• 9 Potential markets for solar thermal cooling systems
• 10 Case studies
• 11 Glossary
• 12 Sources of further information
• References
• Index