1. How Solar Energy Works

• Introduction

2. How Solar Desalination Works

• Energy saving in water desalination techniques

3. How solar space heating works

1. How Solar Energy Works

Part 1- How photovoltaic works

Introduction

The ways to utilize solar energy are categorized into active solar and passive solar. Passive solar systems employ natural processes to transfer the collected solar energy into consumptive targets while active solar systems are equipped to appropriate devices and instruments in order to collect, convert, store and control solar energy for generation of either electricity or heat.

Photovoltaic systems

Photovoltaic modules installed on the roof of a traditional building in the Middle East

Some systems convert the solar irradiation directly into electricity. Photovoltaic (PV) system is the most popular among them with the widest application and fastest annual growth rate (48% per year during the last 8 years) between all energy technologies. The main core of each Photovoltaic system is the PV module which in turn consists of several solar cells that are mounted in a frame. Every PV system employs one or more PV module relevant to its size and power capacity.

A solar cell comprises a light sensitive chip, a transparent cover, a conduction grid, a conductive support sheet and an insulator support. Every light sensitive chip is made from a special category of materials named as semi-conductors. The most popular semi-conductor applied in solar cells is silicon (Si). Silicon atom has 14 electrons spinning in three electron shells around the nucleus. The outer shell has two pairs of electron while there are 4 places for electron pairs from which, two places are empty for another four electrons. Therefore, a silicon atom shares these free places with another four neighbor atoms to make electron bonds in order to form a crystalline structure.

Orbital model of silicon (Si) atom

In order to create conductivity in a crystalline silicon, adding specific impurities is carried on that is called also “doping process”. Silicon doping with phosphorous leaves surplus electrons behind in the crystalline structure because phosphorous atom has five electrons in its outer shell. This kind of doped silicon is called as negative or N-type silicon. Vice versa, doping with boron (B) which has only three electrons in outer shell creates free holes in silicon and makes it as positive of P-type silicon. Inducing external energy such as heat or light into doped silicon can release some free electrons to move around and search for free holes to settle down.
When a N-type and a P-type silicon are joined, an electric field is created at the junction because electrons and holes mix and form a barrier that allows electrons to move from  P-side to N-side only. Electron-hole pairs are released by hitting of light photons (in other words, the energy of radiation) into a thin wafer of bipolar silicon junction. The electrical field sends free electrons to the N-side and free holes to the P-side. Hence, an external conduction path will enable electrons to return to their original place (P-side). The flow of electrons in this path creates the electrical current and the electrical field supplies a voltage. Current and voltage create power together which provides us the electrical energy. Arranging a number of solar cells (usually 36) in series and parallel within a supportive frame makes a PV module with appropriate voltage and current.

General structure of a solar PV cell

Every PV module has its own electrical attribution in the form of voltage-current diagram and consequently has a nominal peak power.

Silicon in solar cells is available in the form of mono crystalline or poly crystalline that is cheaper than mono crystalline but has lower efficiency. The recent efforts have succeeded to produce Solar cells from amorphous silicon which has even lower cost. Beyond silicon, some other types of materials are applied in manufacturing of solar cells such as Cadmium-telluride, Galium-arsenid-phosphid.

A comprehensive PV system consists of the following essential items: A) PV modules, B) batteries, C) battery charger, and optional items such as inverter (for conversion of direct current to alternative current ) and solar tracker for adjustment of PV modules in optimal directions to the sun to receive most solar radiations during the different hours of the day and times of the year.

2. How Solar Desalination Works

• Energy saving in water desalination techniques 

Further to photovoltaic desalination technologies there are solar thermal assisted alternatives to utilize energy from sunshine in order to exploit fresh water from brackish or seawater.

The simplest method is utilization of solar energy in a evaporation-condensation cycle that is also called humidification-dehumidification (HDH) technique through which, solar energy heats the brackish water and makes the water molecules evaporated. The water vapor is consecutively condensed on surfaces with lower temperature such as the internal walls of a transparent collector box where condensate droplets are channeled into a freshwater collector as shown in the image below.

A pilot of HDH solar desalinator at the Solar Institute Jülich, Germany

In order to better recovery of solar energy, a more sophisticated technology has been innovated that is known as multi-effect humidification (MEH) technique in which, multiple evaporation-condensation stages are applied at different temperature levels. This technology provides more efficiency in solar energy consumption. As shown in the picture below, brackish water or sea water is passed through a coil by the help of a pump. The pump can also be driven by power from a solar PV system. This coil acts as a heat exchanger and absorbs the thermal energy from water vapor produced from evaporation of heated saline water. The water that has been partly warmed through the heat exchanger coil enters another heat exchanger in a solar collector system that converts the energy of sunshine into the effective heat. The hot saline water is sprayed then into a chamber in which hot water drips evaporate. As mentioned before, the water vapor exchanges its energy to the heat exchanger coil and consequently converts into salt-free water droplet on the external surface of the heat exchanger that is called also condenser. The condensed water droplets are collected from condenser surfaces properly and channeled into a fresh water storage tank. The un-vaporized brine water is in-turn disposed to the sea or another disposal area.

The highly heat absorbent and corrosion protected materials are key parameter in the efficiency and good performance of this system.

Schematic diagram of MEH technology

Commercial systems of MEH solar desalination units have successfully supplied in various capacities from 500 to 10,000 liters per day around the world by MAGE-TiNox advanced technology.  As a case example, the MAGE Midi-SAL system supplies 5000 L/d drinking water for Jeddah Aviation Club in Saudi Arabia using an array of solar collectors by total surface of 140m² as shown in below picture. More detailed information about these systems is available in www.tinox-watermanagement.de

The solar MEH water desalination system in Jeddah (photo courtesy of MAGE Water Management)

A sate of the art technology of water desalination has recently examined at the pilot scale in Bahrain, named as natural vacuum desalination (NVD) that utilizes renewable energy resources such as solar energy, wind power, energy from wastes or waste heat from industries. In this technology water is evaporated in a vacuum circumstance with much lower energy consumption than conventional distillation techniques. The NVD system comprises a ∏ shape tube in which, one of the vertical parts acts as evaporator, the other as condenser and the horizontal part acts as the water vapor transfer chamber. The elevation difference between free water level and water level inside the evaporation/condensation columns is 10.33m (equal to 1 atm.). Ayhan, Al-Madani and Midilli who developed prototype design and operated the pilot plant, have proposed a renewable energy assisted desalination system for installation in Bahrain on the shore of Persian Gulf where a plenty of annual solar hours and solar irradiation is available. They obtained a distilled water production rate of 0.1798 liter per minute by a tube with a diameter of 1.0 meter.

The schematic principle of the system is shown in the following figure. The sea water is heated by a variety of techniques such as solar collectors, electricity from wind turbine or heat from wastes. It will consequently be boiled in the vacuum evaporation column at much lower temperature than 100°C. The steam will be transferred via a convection fan to the condensation column which is cooled by circulation of cool sea water. Finally the distilled water is transferred to a storage chamber. The details of this system are available at their academic articles mentioned as references below.

Schematic drawing of marine natural vacuum desalination system

References

T. Ayhan, H. Al Madani / “Feasibility study of renewable energy powered seawater desalination technology using natural vacuum technique”- Journal of Renewable Energy 35 (2010) 506–514

A. MIDILLI and T. AYHAN – “Natural vacuum distillation technique, part II: Experimental investigations”- Int. Journal of Energy Research 2004; 28:373–389

3. How solar space heating works

Passive solar space heating systems employ windows, thermal masses, openings and ducts to conduct heat from sunlight and lead warm air into the indoor space of buildings. We can use solar energy for our residential spaces in a couple of passive ways. The simplest way is design and installation of appropriate windows and shadings which prepare shadows during hot summer days and conduct sunlight inside the rooms during winter days. Passive solar space heating systems can be classified into five categories.

1-     Direct gain

2-     Thermal storage walls

3-     Thermal storage roofs

4-     Attached sunspaces

5-     Convective loops

Direct gain is carried out through an architectural design of windows and openings and shadings for allowing the sunlight to enter the living space during winter sunny days while avoiding sunshine entrance into indoor space on hot summer days.

Solar space heating by thermal storage wall employs a chamber with an absorptive thermal mass wall and openings in the back wall. The sunlight irradiates on the surface of front wall which is dark-colored, made from radiance absorptive materials and have a glass cover in front for insulation and reduction of reflective irradiation. The front wall becomes warm against the sunlight and stores heat while in turn warms the air behind itself in the chamber. The warm air is allowed to flow inside the room through the openings. The roof and external walls of the building should be well insulated in order to minimize the heat loss. A schematic of this system has been shown in figure 1. A good manual for design and selection of thermal storage walls has been written by Alex Wilson that is available at the following link: http://nmsea.org/lib/ThermalStorageWallDesignManual.pdf

A thermal storage wall may be built by concrete or masonry materials (brick or rock and mortar) with black or dark colored face against the sunshine. Besides thermal storage wall we can also employ thermal storage masses such as rock bin, water container or brick platform.

Figure 1.                       Simple model of passive solar system with a storage mass wall

Sunspaces are structural elements to capture the solar energy in a passive way. They comprise large surface of glass as well as large area of heat storage mass (concrete stab, brick wall or water container) that are painted in dark colors. Sunspaces should be built in southern side of buildings facing the sunlight (in northern hemisphere). The thermal storage masses exchange their stored heat to the interior space of building gradually via conduction and convection. A better performance in a sunspace can be achieved by double glazing transparent coverage as well as a horizontal reflector for winter days.

Figure 2.                       A typical illustration of attached sunspace for a building

Solar energy can be utilized in a combinative air and water heating system as shown in figure 3. In this system solar collectors are installed on the southern roof of building. Air can either move through the collector to be pre-heated or warmed in a heat-exchanging chamber in which, coils containing hot thermal fluid coming from solar collector have been embedded. Another coil in a hot water tank is connected also to heating fluid circuit of solar collector. Secondary heat exchanger will be inserted in hot water tank supplying the heat from an auxiliary device such as electrical water heater or fuel burner. The warm air after heating by heat exchanger coils will flow to living spaces either by fans or natural convection. The hot water will also be supplied from hot water tank.

Figure 3.                       A typical solar space/water heating system

More useful details on solar space heating are available through the following links

http://www.builditsolar.com/Projects/SolarHomes/PasSolEnergyBk/PSEbook.htm

http://www.builditsolar.com/Projects/SpaceHeating/SolarHeatingIntro15772.pdf