Photovoltaics

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material.

A photovoltaic system  is an arrangement of components designed to supply usable electric power for a variety of purposes, using the Sun (or, less commonly, other light sources) as the power source. Solar PV total global capacity increased during 2010-2013 from 40 GW to 139 GW. In 2013 Germany had the most capacity (36 GW) .

• Off-grid without battery (array-direct)
• Off-grid with battery storage for DC-only appliances
• Off-grid with battery storage for AC and DC appliances
• Grid-tie without battery
• Grid-tie with battery storage

A small PV system is capable of providing enough AC electricity to power a single home, or even an isolated device in the form of AC or DC electric. For example, military and civilian Earth observation satellites, street lights, construction and traffic signs, electric cars, solar-powered tents, and electric aircraft may contain integrated photovoltaic systems to provide a primary or auxiliary power source in the form of AC or DC power, depending on the design and power demands.

The average family home needs a solar PV panel that provides about 3kW of electricity. This will cost between £4,000 and £6,000 and cover about 21m² of roof space.

While this may seem like a large sum you will actually make this back in about ten years because the government pays you for the electricity you produce and you save money on your energy bills.

The government payout is called the Feed-in tariff and it lasts for twenty years.

The price of a solar panel will vary depending on the quality and size .

Solar PV needs little maintenance – you'll just need to keep the panels relatively clean and make sure trees don't begin to overshadow them. In the UK panels that are tilted at 15° or more have the additional benefit of being cleaned by rainfall to ensure optimal performance. Debris is more likely to accumulate if you have ground mounted panels.If dust, debris, snow or bird droppings are a problem they should be removed with warm water (and perhaps some washing-up liquid or something similar – your installer can advise) and a brush or a high pressure hose (or telescopic cleaning pole) if the panels are difficult to reach.

Solar energy

Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermalcollectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.

The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet.

Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. By photosynthesis green plants convert solar energy into chemical energy, which produces food, wood and the biomass from which fossil fuels are derived .

Applications of solar technology :

Solar energy refers primarily to the use of solar radiation for practical ends. However, all renewable energies, other than geothermal and tidal, derive their energy from the sun.

Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies .

• Architecture and urban planning
• Agriculture and horticulture
• Transport and reconnaissance
• Solar thermal
• Water heating
• Heating, cooling and ventilation
• Water treatment
• Process heat
• Cooking
• Electricity production
• Concentrated solar power
• Photovoltaics

CSP development

Photovoltaics (PV) is the dominant solar technology with more than 12 GW installed in 2010 alone, but PV (not to mention wind power) has inherent limitations of intermittency. This gives CSP a distinct advantage, and current advances in heat transfer and storage could increase the implementation of CSP as a significant renewable resource.

Introduction to CSP :

For concentrating solar power (CSP) there are four different technology approaches – parabolic trough, power tower, linear reflector, and sterling dish . Currently, more than 90% of installations use parabolic troughs to generate electricity.

The main advantage of CSP against other sources of renewable energy such as PV or wind is the capability to provide dispatchable power – by storing solar energy in thermal reservoirs and releasing it as and when it's needed i.e. during periods of peak power demand, during cloudy weather or even at night. So storage can eliminate intermittency as well as extend energy production past sun-set. Another advantage to CSP is that it can be used as part of a hybrid energy source, in that a regular gas fired plant can be used to heat the HTF/TES materials (see below) in the event of solar down time. This configuration is much like the early hybrid cars.

Despite these advantages however, CSP-derived electricity is expensive in comparison with other technologies. The key to CSP's commercial success remains in developing an economical and effective energy storage capability.

There are two intertwined technology paths for CSP which both need to be advanced – the solar collection technology and the heat conduction technology. In this article we will address the latter.

Developments in heat transfer and storage materials

Heat conduction technology :

Except for sterling dish technology, CSP needs a Heat Transfer Fluid (HTF), and in some cases a Thermal Energy Storage (TES) medium. The HTF and TES materials are the interface between the solar energy input and the power block.

From a commercial perspective HTF and TES are at very early development stages. Although they can function adequately, the current HTFs suffer from significant shortcomings. As far as the TES elements, this is even more challenging, with only marginal industrial activities underway.

But there is now a strong push to extend the capabilities of HTF/TES, and the results could help enable the acceleration of CSP beyond the 500 MW of current installations. Activity for both remains very much an R&D effort, especially for TES. But there is now an increased emphasis to address both the limitations of current HTF/TES, as well as to develop advanced elements. Groundbreaking research is looking at HTF and TES – for example how nanostructures can be used to tailor the fluids' thermo-physical properties?

New funding is also being directed towards a broad range of near-term improvements (as well as long-range R&D).

The key considerations for improvement of HTF/TES materials include the ability to vary the operational temperatures; the range of useful temperatures; the heat capacitance; and – very importantly – the cost. The latter is essential in the world of solar energy. The rapid implementation of PV in the last year has been accelerated by a drop in PV prices. It is essential for CSP to achieve some of the same cost reductions .

Current limitations to HTF/TES :

Up to now, CSP plants have used synthetic oils as heat transfer fluids and molten salts for thermal energy storage (molten salt can also be an HTF). .

However there are significant limitations which impact the overall cost of CSP electricity. The most proven and commonly used CSP technology is parabolic trough technology, which absorbs solar radiation and reaches temperatures of around 700°F (371°C). In the heat exchanger, water is preheated, evaporated, and superheated into steam, which runs a steam turbine. The water is cooled, condensed, and reused in the heat exchangers. Most of these plants have little or no storage and use oils which are flammable. Operating at higher temperatures enables higher power cycle efficiencies to be achieved .

The organic oil-based HTF currently tend to break down at high temperatures (around 400°C), which prevents solar thermal plants from running at maximum efficiency. Inorganic materials such as salts, on the other hand, maintain stability at high temperatures, but then solidify easily at temperatures as high as 230°C – a problem because when the sun drops in the desert, so does the temperature.

Solar Towers

 In the ST plants, a large number of computer assisted mirrors (heliostats) track the sun individually over two axes and concentrate the solar irradiation onto a single receiver mounted on top of a central tower where the solar heat drives a thermodynamic cycle and generates electricity. In principle, ST plants can achieve higher temperatures than PT and FR systems because they have higher concentration factors.

The ST plants can use water-steam (DSG), synthetic oil or molten salt as the primary heat transfer fluid. The use of high-temperature gas is also being considered. Direct steam generation (DSG) in the receiver eliminates the need for a heat exchanger between the primary heat transfer fluid (e.g. molten salt) and the steam cycle, but makes thermal storage more difficult. Depending on the primary heat transfer fluid and the receiver design, maximum operating temperatures may range from 250-300°C (using water-steam) to 390°C (using synthetic oil) and up to 565°C (using molten salt). Temperatures above 800°C can be obtained using gases. The temperature level of the primary heat transfer fluid determines the operating conditions (i.e. sub-critical, super-critical or ultra-super-critical) of the steam cycle in the conventional part of the power plant.

ST plants can be equipped with thermal storage systems whose operating temperatures also depend on the primary heat transfer fluid. Today’s best performance is obtained using molten salt at 565°C for either heat transfer or storage purposes. This enables efficient and cheap heat storage and the use of efficient super-critical steam cycles. High-temperature ST plants offer potential advantages over other CSP technologies in terms of efficiency, heat storage, performance, capacity factors and costs. In the long run, they could provide the cheapest CSP electricity, but more commercial experience is needed to confirm these expectations

Current installed capacity includes the PS10 and PS20 demonstration projects (i.e. Spain) with capacities of 11 MW and 20 MW, respectively. Both plants are equipped with a 30-60 minute steam-based thermal storage to ensure power production despite varying solar radiation .
The PS10 consists of 624 heliostats over 75,000 m2

 Its receiver converts 92% of solar energy into saturated steam at 250°C and generates 24.3 GWh a year (i.e. 25% capacity factor), with 17% efficiency. In Spain, a 19-MW molten salt-based ST plant
with a 15-hour molten salt storage system started operation in the second half of 2011. It is expected to run for almost 6,500 operation hours per year, reaching a 74% capacity actor and producing fully dispatchable electricity.
Larger ST plants are under construction (e.g. the 370-MW Ivanpah project in California with water-steam at 565°C and 29% efficiency and the 50-MW Supcon project in China) or under development (e.g. eight units with a total capacity of 1.5 GW in the southwestern United States). Large plants have expansive solar fields with a high number of heliostats and a greater distance between them and the central receiver. This results in more optical losses, atmospheric absorption and angular deviation due to mirror and sun-tracking imperfections.

Fresnel Reflectors

FR plants are similar to PT plants but use a series of ground-based, flat or slightly curved mirrors placed at different angles to concentrate the sunlight onto a fixed receiver located several meters above the mirror field.

Each line of mirrors is equipped with a single axis tracking system to concentrate the sunlight onto the fixed receiver. The receiver consists of a long, selectively-coated tube where flowing water is
converted into saturated steam (DSG or Direct Steam Generation). Since the focal line in the FR plant can be distorted by astigmatism, a secondary mirror is placed above the receiver to refocus the sun’s rays. As an alternative, multi-tube receivers can be used to capture sunlight with no secondary mirror. The main advantages of FR compared to PT systems are the lower cost of ground-based mirrors and solar collectors (including structural supports and assembly).

While the optical efficiency of the FR system is lower than that of the PT systems (i.e. higher optical losses), the relative simplicity of the plant translates into lower manufacturing and installation costs compared to PT plants.

However, it is not clear whether FR electricity is cheaper than that from PT plants. In addition, as FR systems use direct steam generation, thermal energy storage is likely to be more challenging and expensive.

FR is the most recent CSP technology with only a few plants in operation (e.g. 1.4 MW in Spain, 5 MW in Australia and a new 30-MW power plant, the Puerto Errado 2, in Spain, which started operation in September 2012). Further FR plants are currently under construction (e.g. Kogan Creek, Australia 44 MW, 2013) or consideration.

Parabolic Trough

PT is the most mature CSP technology, accounting for more than 90% of the currently installed CSP capacity., It is based on parabolic mirrors that concentrate the sun’s rays on heat receivers (i.e. steel tubes) placed on the focal line. Receivers have a special coating to maximize energy absorption and minimize infrared re-irradiation and work in an evacuated glass envelope to avoid convection heat losses .

The solar heat is removed by a heat transfer fluid (e.g. synthetic oil, molten salt) flowing in the receiver tube and transferred to a steam generator to produce the super-heated steam that runs the turbine. Mirrors and receivers track the sun’s path along a single axis (usually East to West). An array of
mirrors can be up to 100 meters long with a curved aperture of 5-6 meters.

Most PT plants currently in operation have capacities between 14-80 MWe, efficiencies of around 14-16% (i.e. the ratio of solar irradiance power to net electric output) and maximum operating temperatures of 390°C, which is limited by the degradation of synthetic oil used for heat transfer. The use of molten salt at
550°C for either heat transfer or storage purposes is under demonstration. High temperature molten salt may increase both plant efficiency (e.g. 15%-17%) and thermal storage capacity. In addition to the SEGS project (i.e. nine units with a total capacity of 354 MW in operation since the 1980s--, major and more recent PT projects in operation include two 70-MW units in the United States (i.e. Nevada Solar One and MNGSECFlorida), about thirty 50-MW units in Spain and smaller units in a number of other countries .

 The three 50-MW Andasol units by ACS/Cobra Group and Marquesado Solar SL and the two 50-MW (Valle I and II) plants by Torresol Energy in Spain are particularly interesting, as they use synthetic oil as the heat transfer fluid and molten salt as the thermal storage fluid. They have a thermal storage capacity of
around 7.5 hours, which can raise the capacity factor up to 40%. In Italy, a 5-MW demonstration plant (ENEL, ENEA) with eight hours of thermal storage started operation in June 2010 to test the use of molten salt as either heat transfer or storage fluid, which can significantly improve the storage performance and the
capacity factor (by up to 50%) because the higher operation temperature and thermal capacity of molten salt enable more storage capacity with reduced storage volume and costs .

 Large PT plants under construction include the Mojave project (a 250 MW plant in California due to start operation in 2013), the 280 MW Solana project in Arizona due in 2013, the Shams 1 100MW project in the United Arab Emirates due in 2012/2013), the Godawari project (India, 50 MW, 2013) and a further fifteen 50-MW plants in Spain.

Concentrating solar power

The limited supply of fossil hydrocarbon resources and the negative impact of CO2 emissions on the global environment dictate the increasing usage of renewable energy sources. Concentrated solar power (CSP) is the most likely candidate for providing the majority of this renewable energy, because it is amongst the most
cost-effective renewable electricity technologies and because its supply is
not restricted if the energy generated is transported from the world's solar belt to the population centers.

Three main technologies have been identified during the past decades for generating electricity :
● dish/engine technology,

which can directly generate electricity in isolated locations
● parabolic and Fresnel trough technology,

 which produces high pressure superheated steam
● solar tower technology,

which produces air above 1000°C or synthesis gas for gas turbine operation.

While these technologies have reached a certain maturity, as has been demonstrated in pilot projects in Israel,Spain and the USA, significant improvements in the thermo-hydraulic performance are still required if such installations are to achieve the reliability and effectiveness of conventional power plants. This first article focuses on present CSP technologies, their history and the state of the art. The second article, in the next issue of Ingenia, looks at the technical,environmental, social and economic issues relating to CSP in the future.

Concentrating Solar Power
Concentrating Solar Power (CSP) is electricity generated from mirrors to focus sunlight onto a receiver that captures the sun’s energy and converts it into heat that can run a standard turbine generator or engine. CSP systems range from remote power systems as small as a few kilowatts up to grid-connected power plants of
100’s of megawatts (MW). CSP systems work best in bright, sunny locations like the Southwest. Because of the economies of scale and cost of operation and maintenance, CSP technology works best
in large power plants.

Why CSP?
- Clean, reliable power from domestic renewable energy
- Operate at high annual efficiency Firm power delivery when integrated with thermal storage
- Easily integrated into the power grid
- Boosts national economy by creating many new solar companies and jobs.

How They Work?
Parabolic trough solar systems use long, parabolic-shaped mirrors or linear Fresnel reflectors to collect and focus sunlight onto a receiver tube that contains a fluid. The fluid inside the tube is heated to create superheated steam that powers a turbine generator to produce electricity.
Parabolic Trough Collector -The sun’s energy is concentrated on an oil-filled, solar absorbing transparent
glass tube running along the focal line of the parabolically shaped trough.
Linear Fresnel Reflectors - Differ from parabolic trough in that the absorber is fixed in space above the
slightly curved or flat Fresnel reflectors. Sometimes a small parabolic mirror is added to the top of the receiver to further focus sunlight.

Advantages of sustainable energy action planning

 Improvements in local air quality
Energy management initiatives are among the most cost effective actions that local authorities can take to reduce the air pollution that causes serious environmental and health problems within their cities.

 Financial savings 
While many local authorities are faced with budget deficits, the appeal of saving money is often the starting
point for municipal energy management initiatives. Improved efficiency in municipal energy consumption
offer plentiful opportunities for reducing operating costs.

 New jobs
Inefficient energy systems represent important investment opportunities in the community and such investments are among the most effective ways to create new employment. When energy management reduces expenditures on fuel and electricity, the savings can then be re-spent within the community.

 Local economic development
The energy management industry itself is a growth industry and its promotion can be an effective component of local economic development strategies in the community. In addition, big business is increasingly considering the livability of a city an important factor in deciding where to locate – access to urban goods and transport efficiency (and  so  spatial  development  and  public  transport provision) are critical to creating livable cities.

 New partnerships
Utilities, private enterprises, financial institutions and levels of government other than municipal are all pursuing energy management for various reasons. They have recognized that urban governments are well suited to deliver the type of integrated programmes often required to achieve energy-efficiency and renewable energy objectives.

What is the future for energy and development ?

The global energy crisis coupled with the threats of climate change bring into sharp focus both opportunities and challenges for developing countries.
 Developing countries have to address the increasing energy demands of growing economies, as well as address energy poverty issues often highlighted by extreme disparities in income. They also need to deal with the real and potential impacts of climate change.
 In addition to these challenges is the global imperative to reduce carbon emissions in order to prevent climate change. While developing nations have thus far been sheltered from obligations to reduce carbon emissions, we cannot anticipate that this situation will continue. Within this context developing nations need to follow a very different development path from that established by first world countries.
 This development path is a low energy, low carbon and generally a resource efficient one. Economies across the world need to change the assumptions of this paradigm in order to build a sustainable reality. As financial and environmental impacts soar, the real costs of resource inputs and of waste generation need to increasingly be taken into account. These factors are making efficiency, conservation, reuse, recycling and renewable energy sources primary considerations for a healthy economy. In an attempt to reduce resource inputs and environmental impacts, some developed nations have managed to ‘decouple’ economic growth from energy consumption essentially resulting in energy inputs that decrease with economic growth.
 This has been achieved through technology and behavior change to improve efficiency and by closing the energy loop in production (e.g. recapturing heat energy released in the production process to then power production).
 Energy-poor countries, such as Japan, have been very successful at achieving this. Implementation of high energy-efficiency and the use of renewable resources are also evident in energy-poor developing countries such as the island states of Reunion and Mauritius.
 As such, under conditions of necessity, pursuing efficient and renewable energy paths is possible. There is potential to greatly improve energy efficiency and
reduce carbon emissions in many upper-middle income developing countries which have a substantial industrial base. For example, South Africa produces a mere US$1.06 in economic value for every 1 kWh of electricity consumed whereas Brazil manages twice and Mexico four times this level of energy efficiency.

What is happening in our cities?
Over the last 20 years, urban centers have experienced dramatic growth. Today half of the world’s total population (around three billion people) live in urban settlements. Developing countries in particular are undergoing rapid change from rural to urban-based economies as they are transformed by their urbanising populations. There are marked differences in the level and pace of urbanisation within less developed regions of the world. Latin America and the Caribbean are highly urbanised, with 78% of their populations living in cities in 2007. Asia and Africa are less urbanised, both with around 40% of their populations living in urban areas. While currently less urbanised, Africa and Asia are experiencing rapid rates of urbanisation.
Consequently by 2050, about 62% of their inhabitants will live in urban areas. At that time, 89% of the population of Latin America and the Caribbean will be urban.

In addition, over the next 30 years population growth will be nearly entirely concentrated in urban areas in the developing world. Much of the current debate regarding sustainable cities focuses on the formidable problems for the world’s largest urban agglomerations. However, smaller urban settlements are also growing rapidly and the majority of all urban dwellers reside in such smaller urban centers.

There is potential to greatly improve energy efficiency and reduce carbon emissions in many upper-middle income developing countries which have a substantial industrial base. For example, South Africa produces a mere US$1.06 in economic value for every 1 kWh of electricity consumed whereas Brazil manages twice and Mexico four times this level of energy efficiency