Publisher: Earthscan, UK Homepage:

Publisher: Earthscan, UK Homepage: www.earthscan.co.uk/?tabid=101808
Energy and the New Reality, Volume 2: C-Free Energy Supply Chapter 2: Solar Energy L. D. Danny Harvey Publisher: Earthscan, UK Homepage: This material is intended for use in lectures, presentations and as handouts to students, and is provided in Powerpoint format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see for contact details.

Framework Solar flux density on a plane perpendicular to the sun’s rays at the mean Earth-Sun distance, Qs, is 1370 W/m2 The intercepted solar radiation flux (Qs x πRe2) is about times the 2005 world primary power demand of 15.3 TW About 0.8% of the world’s desert area (or 80,700 km2) covered with 10% efficient modules would be all that is required to generate the total world electricity consumption in 2005 of about TWh However, cumulative installation of PV panels to date is only 25 km2 The solution is to directly use solar energy where-ever possible (for passive heating and ventilation, for thermal-driven cooling, and for daylighting), and to use solar electricity only where electricity really is needed.

This chapter discusses:
Photovoltaic generation of electricity Solar-thermal generation of electricity Solar thermal energy for space heating and for hot water Solar thermal energy for air conditioning Industrial uses of solar thermal energy Direct uses of solar energy for desalination, in agriculture and for cooking

Chapter 4 (Buildings) of Volume 1 discusses passive (as opposed to active) uses of solar energy, with the building itself serving as a collector of solar energy. These passive uses are Passive heating Passive ventilation Daylighting

Chapter 11 (Community-Integrated Energy systems with Renewable Energy) of this volume discusses seasonal underground storage of solar thermal energy for space heating and for domestic hot water

Figure 2.1a Stereographic sun path diagram
Source: Computed using The Solar Tool developed by Square One Research, available through Ecotech (ecotech.com)

Figure 2.1b Stereographic sun path diagram

Figure 2.1c Stereographic sun path diagram

Figure 2.1d Stereographic sun path diagram

Figure 2.2a Solar irradiance, daily variation, clear sky

Figure 2.2b Solar irradiance, daily variation , clear sky

Figure 2.2c Solar irradiance, daily variation, clear sky

Figure 2.2d Solar irradiance, daily variation, clear sky

Figure 2.2e Solar irradiance, daily variation, clear sky

Figure 2.2f Solar irradiance, daily variation, clear sky

Figure 2.2g Solar irradiance, daily variation, clear sky

Figure 2.2h Solar irradiance, daily variation, clear sky

Figure 2.2i Solar irradiance, daily variation, clear sky

Figure 2.3a Solar irradiance, annual variation, clear sky

Figure 2.3b Solar irradiance, annual variation, clear sky

Figure 2.3c Solar irradiance, annual variation, clear sky

Figure 2.4a Solar irradiance on windows in June, clear sky

Figure 2.4b Solar irradiance on windows in December, clear sky

Figure 2.5 Annual average solar irradiance (W/m2) at ground level on a horizontal surface
Source: Henderson-Sellers and Robinson (1986, Contemporary Climatology, Longman, Harlow, U.K)

Supplemental Figure: Solar irradiance on a horizontal surface, kWh/m2/yr
Source: Prepared from data file obtained from the NASA Surface Meteorology and Solar Energy website, power.larc.nasa.gov

Supplemental Figure: Solar irradiance on a surface tilted toward the equator at an angle equal to the latitude angle, kWh/m2/yr Source: Prepared from data file obtained from the NASA Surface Meteorology and Solar Energy website, power.larc.nasa.gov

Supplemental Figure: Ratio of annual irradiance on a surface tilted at the latitude angle to the annual irradiance on a horizontal surface Source: Prepared from data files obtained from the NASA Surface Meteorology and Solar Energy website, power.larc.nasa.gov

Supplemental Figure: Ratio of local annual irradiance on a surface tilted at the latitude angle to the maximum annual irradiance on a horizontal surface anywhere Source: Prepared from data files obtained from the NASA Surface Meteorology and Solar Energy website, power.larc.nasa.gov

Two broad ways of making electricity from solar energy:
Photovoltaic (PV) Solar thermal

PV Electricity Electromagnetic radiation (including light) comes in packets called photons, each with energy hv, where h=Plank’s constant and v is the frequency of the radiation Electrons in an atom exist in different energy levels A photon can bump an electron to a higher energy level if the energy of the photon exceeds the difference in energy from one level to the next

PV electricity (continued)
When a solid forms, two outer energy bands are formed, often separated by a gap The lower energy band is called the valence band, the upper the conduction band In a conductor, electrons occur in both bands and they overlap In an insulator, the valence band is filled and the conduction band is empty, and the two bands do not overlap In a semi-conductor, electrons occur in both bands and there is a small gap between the bands

PV electricity (continued)
Silicon is a semi-conductor with a valence of 4 (4 electrons in the outer shell) Two semiconductor layers are used – one layer (called the n-type layer) is doped with atoms have an valence of 5 (the extra electron is not taken up in the crystal lattice and so it free to move), and the other layer (called the p-type layer) is doped with atoms having a valence of 3, so there are empty electron sites (called holes) The juxtaposition of the n- and p layers is called a p-n junction.

Figure 2.6 Steps in the generation of electricity in a photovoltaic cell
Source: US EIA (2007, Solar Explained, Photovoltaics and Electricity)

Figure 2.7 Layout of a silicon solar cell
Source: Boyle (2004, Renewable Energy, Power for a Sustainable Future, , Oxford University Press, Oxford)

Components of a PV system
Module – consists of many cells wired together Support structure Inverter – converts DC module output to AC power at the right voltage and frequency for transfer to the grid Concentrating mirrors or lens for concentrating PV systems

Types of PV cells Single-crystalline Multi-crystalline
Thin-film amorphous silicon Thin-film compound semiconductors Thin-film multi-crystalline Nano-crystalline dye-sensitized cells Plastic cells

Thin-film compound semiconductors
Cadmium telluride (CdTe) Copper-indium-diselenide (CuInSe2, CIS) Copper-indium-gallium-diselenide (CIGS) Gallium arsenide (GaAs)

Table 2.3 Best efficiencies achieved as of 2009

Figure 2.8 Trend in efficiency of PV cells and modules
Source: Extended from IEA (2003, Renewables for Power Generation, Status and Prospects, International Energy Agency, Paris)

Figure 2.9. Structure of the GaInP/GaInAs/Ge multi-junction Cell
Source: Kinsey et al (2009, Progress in Photovoltaics: Research and Applications 16, )

Figure 2.10 Organic Semiconductors
Source: Rand et al (2007, Progress in Photovoltaics: Research and Applications 15, 659–676)

Figure 2.11 Dye-sensitized Solar Cell
Source: McConnell (2002, Renewable and Sustainable Energy Reviews 6, 271–295,

Factors affecting module efficiency
Solar irradiance – efficiency peaks at around 500 W/m2 for non-concentrating cells Temperature – efficiency decreases with increasing temperature, more so for c-Si and CIGS, less for a-Si and CdTe Dust – can reduce output by 3-6% in desert areas

Figure 2.12a Module efficiency vs solar irradiance, theoretical calculations
Source: Topic et al (2007, Progress in Photovoltaics: Research and Applications 15, 19–26)

Figure 2.12b Module efficiency vs solar irradiance, measurements
Source: Mondol et al (2007, Progress in Photovoltaics: Research and Applications 15, 353–368)

System efficiency is the product of
Module efficiency Inverter efficiency MPP-tracking efficiency

Figure 2.13a Inverter & MPP Efficiency, calculated

Figure 2.13b Inverter & MPP Efficiency, measured
Source: Mondol et al (2007, Progress in Photovoltaics: Research and Applications 15, 353–368)

Figure 2.14 Current-voltage combinations (MPP) giving the maximum power production for different solar irradiances on the module Source: Hastings and Mørck (2000, Solar Air Systems: A Design Handbook. James & James, London)

Figure 2.15 MPP-tracking efficiency
Source: Abella and Chenlo (2004, Renewable Energy World, vol 7, no 2, pp132–146 )

The net effect of all the losses is represented by the performance ratio: the ratio of actual kWh of generated AC electricity to kWh of DC electricity produced by the module Recent values have averaged around 75-80%

Building-Integrated PV (BiPV)

Figure 2.16 PV mounted onto a sloping roof
Source: Prasad and Snow (2005, Designing with Solar Power: A Sourcebook for Building Integrated Photovoltaics, Earthscan/James & James, London)

Figure 2.17 PV integrated into a sloping roof
Source: Omer et al (2003, Renewable Energy 28, ,

Figure 2.18a BiPV on single-family house in Finland
Source: Hestnes (1999, Solar Energy 67, 181–187,

Figure 2.18b BiPV on a single-family house in Maine
Source: Hestnes (1999, Solar Energy 67, 181–187,

Supplemental figure: BiPV on multi-unit housing somewhere in Europe

Figure 2.19 PV modules (attached to insulation) on a horizontal flat roof
Source:

Figure 2.21 BiPV (opaque elements) on the Condé Nast building in New York
Source: Eiffert and Kiss (2000, Building-Integrated Photovoltaic Designs for Commercial and Institutional Structures: A Sourcebook for Architects, National Renewable Energy Laboratory, Golden, Colorado)

Figure 2.22 PV modules servings as shading louvres on the Netherlands Energy Research Foundation building Source: Photographs by Marcel von Kerckhoven, BEAR Architecten (

Supplemental figure PV modules as vertical shading louvres on the SBIC East head office building in Tokyo Source: Shinkenchiku-Sha and jpn_02.htm

Figure 2.23 PV modules providing partial shading in the atrium of the Brundtland Centre (Denmark, left) and Kowa Elementary School (Tokyo, right) Source: Shinkenchiku-Sha Source: Henrik Sorensen, Esbensen Consulting

Supplemental figure: Amersfoort project, The Netherlands

Table 2.4: Potential electricity production from BiPV
Source: Gutschner and Task 7 Members (2001, Potential for Building-Integrated Photovoltaics,

Parking lots in the US: Area of 1.9 million ha (19000 km2, or km x km) PV covering all parking lots at 180 W/m2 and 15% efficiency would generate ~ 4500 TWh/yr Total US electricity demand is ~ 4200 TWh/yr

Concentrating PV More sunlight on the expensive solar cell (by up to a factor of 500), using less expensive mirrors or lens Cell efficiencies are greater under concentrated sunlight, compounding the benefit of greater solar irradiance Works only with direct irradiance (not diffuse) Requires 1- or 2-axis sun tracking Passive or active heat removal required

Figure 2.24 Concentrating PV using a Fresnel lens
Source:

Figure 2.25 Entech concentrating PV
Source:

Figure 2.26 Amonix concentrating PV
Source:

Figure 2.27 Flatcon point focus concentrating PV
Source: Peharz and Dimroth (2005, Progress in Photovoltaics: Research and Applications 13, 627–634)

Figure 2.28a Growth in annual PV production

Figure 2.28b Growth in installed PV power

Cost of PV electricity Module cost per kW peak output
= (module cost per m2 )/ (ηm Ip) where Ip is the assumed maximum irradiance (1000 W/m2) and η m is the module efficiency (sunlight to DC) Electricity cost ($/kWh) = (CRF+INS)*(1+ID)*CapCost/(8760 * CF * ηbos) where CRF and INS are the cost recovery and insurance factors, ID is an indirect factor, CapCost is the total capital cost ($/kWp-DC), CF = Ia/Ip ,ηbos is the balance-of system efficiency, and Ia is the mean annual irradiance

Component and installed costs
Modules: ~ $400/m2, or $4000/kW if the efficiency is 10% Inverters: ~ $ /kW-DC (less for larger systems) Total installed cost: ~ $ /kW

Costs of alternative cladding materials:
Stainless steel: ~ $ /m2 Glass-wall systems: ~ $ /m2 Rough stone: ~ ≥ $750/m2 Polished stone: ~ $ /m2

Table 2. 8 Illustrative costs of PV electricity (cents/kWh(AC)) for 2
Table 2.8 Illustrative costs of PV electricity (cents/kWh(AC)) for 2.5%, 5%, and 10%/yr real financing costs, a 20-year lifespan, 1%/yr insurance, 25% indirect costs, $400/kW (DC) power conditioning, 75% BOS efficiency, and $1.1/m2 per year operation and maintenance costs

Projection of future costs
Extrapolation using the progress ratio concept Engineering-based bottom-up analysis

Figure 2.29 Price of Photovoltaic Module
Source: van Sark et al (2008, Progress in Photovoltaics: Research and Applications 16, )

Results of bottom-up analyses:
Projected near-term (2015) module costs of $1/Wp for both c-Si and a-Si, installed costs of $3/Wp or less With 1 GWp/yr manufacturing facilities

Figure 2.30 Triple-junction a-Si on laminated roofing
Source: Hegedus (2006, Progress in Photovoltaics: Research and Applications 14, 393–411)

Figure 2.31a PV Scenario

Figure 2.31b PV Scenario

Figure 2.32 Computation of cumulative subsidy

Figure 2.33a Cumulative Subsidy

Figure 2.33b Cumulative Subsidy

Resource constraints on thin-film PV
CIGS will be limited by the Indium supply CdTe will be limited by the tellurium supply The constraints involve both the absolute supply of In or Te, and the rate at which it can be supplied In and Te are supplied as a byproduct of mining copper, zinc, and bauxite

In the absence of concentrating PV,
CdTe, CIGS, and a-Si:Ge together are unlikely to be able to provide more than 1 TW peak power (compared to 4.3 TW global electricity generating capacity and 15.3 TW average global primary power demand in 2005) Dye-sensitized cells (which require ruthenium) could provide 6 TWp Near 100% recycling of rare elements would be required for long-term sustainability

Solar Thermal Generation of Electricity
Mirrors are used to concentrate sunlight either onto a line focus or a point focus Steam is generated, and then used in a steam turbine In some cases, concentrated solar energy heats a storage medium (such as molten salt), so electricity can be generated 24 hours per day using stored heat at night Best in desert or semi-desert regions, as only direct-beam solar radiation can be used

The radiation available for use by concentrating solar thermal power (CSTP) systems is referred to as the ‘direct normal’ radiation – the annual value is the irradiance on a surface that is always at 90o to the sun’s rays As only the direct beam radiation can be used, the peak irradiance that can be used by CSTP is typically about 850 W/m2, compared to 1000 W/m2 for PV systems Thus, peak power capacity for CSTP is given assuming a direct beam irradiance of 850 W/m2 rather than 1000 W/m2 The annual capacity factor is equal to the annual average direct normal irradiance (in W/m2) divided by 850 W/m2 (in the same way that the annual capacity factor for PV is given by the annual average irradiance on the module divided by 1000 W/m2)

Supplemental Figure: Annual direct normal irradiance, kWh/m2/yr
Source: Prepared from data files obtained from the NASA Surface Meteorology and Solar Energy website, power.larc.nasa.gov

Supplemental Figure: Ratio of annual direct normal irradiance to annual total irradiance on a horizontal surface Source: Prepared from data files obtained from the NASA Surface Meteorology and Solar Energy website, power.larc.nasa.gov

Types of Solar Thermal Systems:
Parabolic trough Parabolic dish (Stirling engine) Central tower

Figure 2.34a Parabolic trough schematic
Source: Greenpeace (2005, Wind Force 12: A Blueprint to Achieve 12% of the World’s Electricity from Wind Power by 2020, Global Wind Energy Council,

Figure 2.34b Central receiver schematic

Figure 2.34c Parabolic dish schematic

Figure 2.35a Parabolic Trough Thermal Electricity, Kramer Junction, California

Figure 2.35b Parabolic Trough Thermal Electricity, Kramer Junction, California

Figure 2.35c Close-up of parabolic trough

The latest parabolic trough systems either
Directly heat the water that will be used in the steam turbine, or Directly heat water that in turn is circulated through a hot tank of molten salt, with the molten salt storing heat and in turn heating the steam that is used in a steam turbine, as illustrated in the following diagram

Figure 2.36 AndaSol-1 Schematic
Source : Translated from Aringhoff (2002, Proyectos Andasol, Plantas Termosolares de 50 MW’, Presentation at the IEA Solar Paces 62nd Exco Meetings Host Country Day )

With thermal storage, Electricity can be generated 24 hours per day
The capacity factor (average output over peak output) can reach 85%

Figure 2.37 Parabolic trough capacity factor
Source : Price et al (2007, Proceedings of Energy Sustainability 2007, June, Long Beach, California)

Table 2.14 Characteristics of existing and possible future parabolic-trough systems
Source: EC (2007, Concentrating Solar Power, from Research to Implementation, and Solúcar

Figure 2.38 Integrated Solar Combined-Cycle (ISCC) powerplant

Figure 2.39 Parabolic dish/Stirling engine for generation of electricity
Source: US CSP (2002) Status of Major Project Opportunities, presentation at the 2002 Berlin Solar Paces CSP Conference

Figure 2.40 Stirling Receiver
Source: Mancini et al (2003, Journal of Solar Energy Engineering 125, 135–151)

Figure 2.41 Energy flow in 4 different parabolic dish/Stirling engine systems

Figure 2.42 Central tower solar thermal powerplant in California
Source: US CSP (2002) Status of Major Project Opportunities, presentation at the 2002 Berlin Solar Paces CSP Conference

Figure 2.43 Solar Thermal Seasonal variation in the production of solar-thermal electricity in Egypt, Spain, and Germany Source: GAC (2006, Trans-Mediterranean Interconnection for Concentrating Solar Power, Final Report, )

Table Comparison of current performance and current and projected cost of different solar thermal technologies for generating electricity

Figure 2.44 Projected cost of heliostats (accounting at present for half the cost of central-tower systems) vs production rate (starting from present costs and production) Source: IEA (2003, Renewables for Power Generation, Status and Prospects, International Energy Agency, Paris)

Solar Thermal Energy For Heating and for Domestic Hot Water

Figure 2.45 Types of collectors for heating and domestic hot water
Source: Everett (2004, Renewable Energy, Power for a Sustainable Future, 17-64, Oxford University Press, Oxford)

Figure 2.46 Installation of flat-plate solar thermal collectors
Source:

Figure 2.47a Integration of solar thermal collectors into the building facade
Source: Sonnenkraft, Austria

Figure 2.47b Integration of solar thermal collectors into the building roof
Source: Sonnenkraft, Austria

Supplemental figure: Evacuated-tube solar thermal collectors
Source: Posters from the AIRCONTEC Trade Fair, Germany, April 2002, available from

Figure 2.48 Integrated passive evacuated-tube collector and storage tank in China
Source: Morrison et al (2004, Solar Energy 76, ,

Figure 2.49 Compound parabolic-trough solar-thermal collector by Solargenix
Source: Gee et al (2003, 2003 International Solar Energy Conference, Kohala Coast, Hawaii, USA, 15-18 March 2003, )

Efficiency of solar thermal collectors:
This is the ratio of heat energy supplied to solar energy incident on the collector Heat is lost as the collector heats up Thus, the key to high efficiency is to supply lots of heat at a relatively low temperature, through a combination of low inlet water temperature and high flow rate To do this, the end use applications must be able to make use of heat at relatively low temperature

For space heating, this requires being able to use heat as it is generated in a radiant floor heating system, which in turn requires high thermal mass exposed to the inside (so that the building does not overheat) and a high-performance envelope (so that the building stays warm after sunset without having to store solar heat in a hot water tank) For domestic hot water, this requires use of a thermally-stratified hot-water tank, with cold water from the bottom of the tank fed to the solar collector and hot water for use drawn from the top of the collector Phase-change materials (which store heat without a further increase in temperature) can also be used. Materials with melting points around 60-70oC would be ideal for domestic hot water applications.

Figure 2.50 Efficiency of solar thermal collectors

Costs in Europe Solar-air collectors, 200-400 euros/m2
Flat-plate or CPC collectors, euros/m2 Evacuated-tube collectors, euros/m2 Storage system costs are extra

Table 2.17 Illustrative costs of solar thermal energy

Table 2.16: Countries with 1 million m2 or more of solar thermal collectors in 2007.
Source: Weiss et al (2009, Solar Heat Worldwide,

Figure 2.51 Growth in the worldwide area of solar thermal collectors

Figure 2.52a Top ten countries in terms of total solar thermal collector area

Figure 2.52b Top ten countries in terms of solar thermal collector area per capita

System-level interactions with solar domestic hot water
Normally, some back-up hot water heating system is needed with solar thermal systems When solar thermal energy is used, the back-up system on average runs at lower efficiency than if it is the sole source of hot water (efficiency can drop from 85% to 45% if solar provides 80% of the required hot water) Thus, the net savings in energy is less than the fraction of the hot-water load met with solar energy (when 80% of the load is met with solar, the savings could be 80% of that, or 64%) If the backup system is a modulating condensing heater, there will not be an efficiency loss at part load

Solar Thermal Energy For Air Conditioning and Dehumidification
Absorption chillers Solid desiccant systems Liquid desiccant systems

From Chapter 4 of Volume 1: Desiccant cooling systems require heat in order to regenerate the desiccant. The desiccant dehumidifies the supply air, making it sufficient dry that cooling of the supply air through evaporation of water is feasible with producing air that is too humid

Temperature-mixing ratio trajectories with desiccant dehumidification and evaporative cooling

The effectiveness of any cooling system is represented by its Coefficient of Performance (COP), which is the ratio of cooling provided to energy input For conventional electric cooling systems, the COP ranges from 2.0 (low-end room air conditioners) to 7.0 (in large central systems with cooling towers) For absorption chillers, the COP is ~0.6 using 90°C heat and ~ 1.2 using 120°C heat as the energy input For solid desiccants, the COP is ~ 0.5 using 80°C heat For liquid desiccants, the COP is ~ 0.75 using 75°C heat

System considerations with solar air conditioning with absorption chillers:
If fossil fuels are used to produce heat for absorption chillers as a backup when there is not adequate solar heat, the inefficiency of the absorption chiller compared to an electric chiller offsets some of the benefit of moving from an electric chiller to an absorption chiller using solar energy part of the time The low COP of the absorption chiller compared to an electric chillers means that more heat in total needs to be removed by the cooling tower, so the electricity use by auxiliary equipment (fans, motors, pumps) is larger, and this offsets some of the benefit of switching from electricity to solar heat for the core cooling function

Figure 2.53 COP vs Driving Temperature for thermally-driven cooling equipment
Source: Balaras et al (2007, Renewable and Sustainable Energy Reviews 11, 299–314,

Costs 1000 to 8000 euros per kW of cooling capacity for solar thermal systems 100 euros/kW for large conventional cooling systems The cost of solar systems is dominated by the cost of the collectors, so if collector costs come down, or the collectors are used for heating in the winter (so that only part of the collector cost need be ascribed to cooling), then the cooling cost will be smaller

Solar cogeneration Mount a PV module over a solar thermal collector, so that both electricity and useful heat are collected By removing heat from the back of the module, the PV electrical efficiency increases However, the thermal collection efficiency will not be as large as for a dedicated solar thermal collector, and there might be an extra glazing over the PV panel, which reduces the production of electricity by absorbing some solar radiation

Figure 2.54 Cost vs solar collector area for solar-thermal air conditioning
Source: Balaras et al (2007, Renewable and Sustainable Energy Reviews 11, 299–314,

Figure 2.55 Cost of saved primary energy versus the magnitude of the savings

Figure 2.56 Proposed hybrid electric-thermal cooling using parabolic solar collectors

Figure 2.57: Cross-section of a PV/T solar collector
Source: Charalambous et al (2007, Applied Thermal Engineering 27, 275–286,

Industrial uses of Solar Energy
Low temperature (60-260oC) Food processing – often at oC Textiles Some chemical and plastics processes High temperature ( K, readily achieved with solar furnaces) reduction of metal ores

Other uses of solar energy:
Desalination of seawater Fixation of nitrogen Solar cooling of greenhouses (with desiccants and evaporation) Crop drying Cooking

Desalination Options:
Solar electricity to power reverse osmosis desalination (80 MJ solar/m3) Solar heat to power multi-stage flash desalination (600 MJ solar/m3) Extraction from night air with desiccants, regeneration of desiccant with solar heat Condensation from humidified air with cool seawater

Source: Clery (2011, Science 331, 136)

Dealing with intermittency
Use rapidly variable fossil backup Aggregate geographically-dispersed PV arrays Install energy storage (V2G plug-in hybrid cars in particular) and develop dispatchable loads Link diverse renewable energy resources (especially if the variability of non-solar resources complements the solar variability)

Figure 2.58a PV Variability
Source: Wiemken et al (2001, Solar Energy 70, 6, 513–518,

Figure 2.58b PV Variability
Source: Wiemken et al (2001, Solar Energy 70, 6, 513–518,

Figure 2.59 Decreasing correlation between output of PV modules with increasing distance between them Source: Wiemken et al (2001, Solar Energy 70, 6, 513–518,

Concluding Comments The solar energy resource is enormous but diffuse, so large land areas would be involved in capturing it Many of our energy needs involve low-temperature heat (for space heating and hot water, and for some industrial processes), and so do not require the intermediary of expensive solar electricity Thus, the first strategy in using solar energy should be to design buildings to make passive use of solar energy – for heating, ventilation, and cooling (which occurs when passive ventilation brings in outside air that is cooler than the temperature that the building would reach on its own)

Concluding Comments (continued)
Two strategies for generating electricity for solar energy are photovoltaic (PV) and solar-thermal PV electricity can be done centrally or on site as building-integrated PV (BiPV) BiPV alone could provide 15-60% of total electricity needs in various countries Solar thermal electricity can be generated 24 hours per day but requires direct-beam solar radiation – so it is best in desert or semi-arid regions

Concluding Comments (continued)
PV cells can use conventional materials (silicon) or various toxic (As, Cd) or rare (Ge, In, Te, Ru, Se) elements, with those using rare elements being most efficient (up to 30%, vs 10-15% for crystalline silicon-based cells and 6-8% for amorphous silicon) Limits on the availability of the rare materials represent real constraints on how much electricity could be supplied with these cells The limit can be increased by a factor of 100 or so using concentrating PV cells, and would no longer be an issue.

Table 2.21: Summary of methods to produce electricity from solar energy

Concluding comments (continued)
PV electricity is currently expensive (~ cents/kWh in sunny locations, cents/kWh in midlatitude locations) but will likely fall in price by a factor of 2 or more during the next decade This would make BiPV highly competitive with peak electricity, which can cost cents/kWh (retail price) Parabolic-trough concentrating solar-thermal electricity is already in the cents/kWh range and could drop to as low as 5 cents/kWh

Other active uses of solar energy
Solar air conditioning Medium-temperature (60-260oC) industrial heat High-temperature ( oC) industrial heat Solar fixation of nitrogen Crop drying Cooking

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