Solar cell is a current source, the electronic characteristic of which can be represented by an equivalent circuit as shown in figure below.

Equivalent Circuit

An ideal solar cell equivalent circuit consists of a current source (Iph ) and a diode. But in practice there are two extra resistances, one in series (Rs) and the other in parallel (Rsh). The series resistance is because of the fact that a solar cell is not a perfect conductor and represents the ohmic loss. The parallel resistance is caused by the recombination of electron-hole pair or leakage current from one terminal to the other due to poor insulation at edges.

From the equivalent circuit diagram,

Iph – Id – Ish = I

Where,

I_d = I_o*[exp(q*V_d / nkT) – 1]

V_d = V + I*R_s

I_sh = V_d / R_sh

Hence, I_ph – I_o*[exp(q*( V + I*R_s ) / nkT) – 1] – (V + I*R_s ) / R_sh = I

Iph = Current produced by the solar cell       Id = Diode current

I_sh = Shunt or leakage current                      I = Load current

I_o = Reverse saturation current of diode      q = Charge of an electron

V = Voltage across load                               n = Diode quality factor

k = Boltzmann’s constant                            T = Absolute temperature of the cell

Generally, series resistance is of very low value and shunt resistance is of very high value. Ideally, Rs = 0 & Rsh =  ∞. A shunt resistance of a few hundred ohms does not reduce the output power of the solar cell appreciably.  In reality, Rsh is much larger than a few hundred ohms and can in most cases be neglected.  The series resistance, however, can drastically reduce output power.

Short circuit current (I_sc) and Open circuit voltage (V_oc): Two quantities of interest for solar cell i.e. I_sc & V_oc can be calculated from the above equivalent circuit equation by neglecting R_sh .

Putting I = 0, V_oc = (nkT/q)*ln(I_ph/I_o +1)

Putting V = 0, I_sc = I_ph – I_o*[exp(qI_scR_s / nkT) – 1]

If we further neglect R_s i.e. R_s = 0, then

Isc = Iph

Types of wind energy converters:

Before we go into further details about the wind energy systems, let’s see the types of wind energy converters, or turbines. They can be broadly categorized into two types;

(1)   (1)Horizontal-axis wind turbines rotate about an axis that is horizontal. They include:

(a) ‘Dutch-type’ windmills-used mainly for grain grinding

(b) Multi-blade windmills-used for pumping

(c) High-speed propeller type windmills

'Dutch-type' Windmill

Multi-blade Windmill

High-speed Propeller-type Wind turbine

(1) (2)Vertical-axis wind turbines have their rotation axis vertical. They are of two types:

(a) The Savonius rotor

(b) The Darrieus rotor

The Darrieus Rotor

There exists a variant of the Darrieus rotor turbine, called the Giromill. In this the blades are straight rather than ‘troposkein’.

The Giromill

We saw in the previous post that Cp has a maximum value of 16/27. However, it also depends upon the type of turbine used.

Power Coefficient for varous wind converters

Solar Cell Structure and Principle of Operation

Solar cells basically consist of p-type and n-type semiconductors with two metal contacts, one in p-type and the other in n-type as shown in figure. These two types of semiconductors form a p-n junction with n-type facing the sun.

Absorption of Solar Radiation

Sunlight is composed of photons containing energy which correspond to the different wavelengths of the solar spectrum. When photons strike a solar cell, depending upon the type of semiconductor used, some low energy photons (infrared) are not absorbed by the electrons in the semiconductor material while some high energy photons are absorbed for excitation of electrons into the conduction band and the excess of energy is released as heat.These excited electrons are attracted toward the n-type semiconductor. This causes more negative charges in the n-type and more positive charges in the p-type semiconductors. If an electric load is connected between the two types of semiconductors through the metal contacts,electrons start flowing from n-type to p-type semiconductors via the load. Thus photon energy is directly converted to electrical energy without an intermediate mechanical or thermal process.

The cell is a current source of DC type. This DC current is used directly in applications. AC current can also be produced by using inverters.

Goldman, right in the picture and Ned Tozun in the left

On the last Saturday, while attending a talk by Mr. Jishnu Bhattacharjee who is an alumnus of IIT Kgp and a team member of the venture capitalist firm Nexus India, I came across a company named “d.light design”. Nexus India is one of the funding organizations for d.light design. The company d.light design is co-founded by Sam Goldman and Ned Tozun, both are Stanford MBAs. It is one of the growing numbers of start-ups aiming to eliminate the kerosene lamps usage in world especially developing countries via alternative power generation sources. So whats their plan of action and why did I notice them?

d.light design offers light sources using LEDs that can be recharged using solar power or by pluging into a grid. Using Silicon Valley based technology; the d.light product line seems to provide a high quality replacement for kerosene lanterns, candles, and emergency lanterns at an economical range from $7 to $30. They are offering three products at present: Nova, Vega, and Comet. The claim that their products give several times brighter light than a kerosene lantern. These products also have the flexibility of adjusting the brightness level to make the illuminance suite the prevailing condition or need so that unnecessarily power doesn’t get wasted. By offering their products in both solar and AC chargeable versions, they may have a greater market share.

Vega

Nova

Comet

So with all these set of products, which market could they have targeted other than India as their stepping stone. Yes, the company is initially focusing its efforts in India because of the size of the market here In India, their office is situated in Noida and they are getting the lamps manufactured in China. Lets see what they actually deliver and how far do they succeed in their vision of replacing every kerosene lantern in the world with safer, brighter, and more affordable lighting by 2017. Wish them good luck with their business.

Wind Energy-where does it come from?

As was mentioned in the article “Renewable Energy-the present state of affairs”, wind energy is “in vogue”. Basically it uses the kinetic energy of moving air, and converts it into useful electrical or mechanical energy. We will get into the details of it soon, but for now let’s just say that it works, in principle, exactly opposite the way a fan works. You supply electricity to a fan, it provides flow of air. Reverse it, i.e. provide blowing air to a fan, it generates electricity. In reality, however, wind energy is a converted form of solar energy. The sun’s radiation heats different parts of the earth at different rates-most notably during the day and night, but also when different surfaces (for example, water and land) absorb or reflect at different rates. This in turn causes portions of the atmosphere to warm differently. Hot air rises, reducing the atmospheric pressure at the earth’s surface, and cooler air is drawn in to replace it. The result is wind.

Wind power production: National average values for year 2000 are shown, based upon BTM (2001) and an average capacity factor of 0.3. The world average for year 2000 is 0.92 W/cap. The growth in cumulated installed capacity from 2000 to 2001 was 35% (BTM, 2002). Some observers expect the growth to slow during the following years, for economic and political reasons, but then to resume growth (Windpower Monthly, 2003).

How much can we extract?

Wind energy is low quality energy, and hence converting it into a high quality form incurs some expenditure. What is that expenditure? First let’s calculate the energy contained in wind. Let the wind speed be v₁ flowing through a cross-section of area S. then the power contained is:

P = ½ .ρ. (S.v₁).v₁²

= ½.ρ.S.v₁³

where, ρ is the air density.

However, all of this cannot be converted into useful energy. In order to calculate the maximum theoretical efficiency of a thin rotor (of, for example, a wind mill), imagine it to be replaced by a disc that withdraws energy from the fluid passing through it. At a certain distance behind this disc the fluid that has passed through flows with a reduced velocity.

Let v₁ be the speed of the fluid upstream and v₂ the speed downstream. The mean flow velocity through the disc representing the rotor is v_avg, where

V_avg = ½.(v₁ + v₂)

With the area of the disc equal to S, and with ρ = fluid density, the mass flow rate (the mass of fluid flowing per unit time) is given by:

m’ = ρ.S.V_avg = ½.ρ.S.(v₁ + v₂)

The power extracted is the difference between the kinetic energies of the flows approaching and leaving the rotor in unit time:

E’ = ½.m’. ( v₁² - v₂²)

= ¼.ρ.S. (v₁ + v₂). ( v₁² - v₂²)

= ¼. ρ.S. (1- v₁³ ((v₂ / v₁)² + (v₂ / v₁) – (v₂ / v₁)³))

By differentiating E’ with respect to (v₂ / v₁) for a given fluid speed v₁ and a given area S one finds the maximum or minimum value for E’. The result is that E’ reaches maximum value when (v₂ / v₁) reaches 1/3.

Substituting this value results in:

P = (16/27). (1/2).ρ.S.v₁³

The “power coefficient” C_p has a maximum value of: C_p.max = 16/27= 0.593. This number is called the Betz Limit, and it gives the maximum fractional wind energy that can be generated from a certain available wind energy.

Source: http://www.esru.strath.ac.uk/

Fuel Cell System is a complex one including the interactions of mechanical, chemical, and electrochemical subsystems present, the stack being the heart of the system. The control of fuel cell systems under a variety of environmental conditions and over a wide operating range is a crucial factor in making them viable for extensive use in every-day technology. So, it is necessary to understand the heart of the system i.e. the fuel cell stack properly.

Fuel cells are chemical engines that generate electricity by converting the chemical potential of the fuel into electrical power. Since they are not based on temperature differences, they are not subjected to Carnot’s limit of efficiency. In addition, common pollutants such as sulfur dioxide and nitrous oxides are avoided since the process does not involve combustion. These advantages, together with the reduction of greenhouse gases and fuel consumption due to higher efficiencies and the possibility of alternative energy sources, have generated enormous interest in fuel cells for stationary as well as mobile applications.

A fuel cell is a class of galvanic cell based on oxidation-reduction reaction. The most basic system uses pure hydrogen as fuel, which is oxidized at the anode, producing electrons and protons:

Oxidation Reaction: H₂ –> 2H⁺ + 2e^-

The electrons are released to an external circuit, where they can be used to perform work, while the protons diffuse through an electrolyte to the cathode. At the cathode, oxygen reacts with the electrons from the external circuit and protons from the anode reaction, forming water:

Reduction Reaction: 0.5O₂+2H⁺+2e^- –> H2O

Therefore, the overall chemical reaction of the PEMFC is:

Overall Reaction: H₂+0.5 O₂-> H₂0+electricity+heat

Source: REN21-http://ren21.net/

Wind Energy:

Source: World Wind Energy Association-http://www.wwindea.org/home/index.php

One of the earliest resources of energy known to man, wind energy is coming to man’s rescue. In the year 2007, 19.696 MW of new wind energy capacity were added summing up to a global installed capacity of 93.849 MW by the end of December 2007. The added capacity equals a growth rate of 26.6 %, after 25.6 % in 2006. The currently installed wind power capacity generates 200 TWh per year; equaling 1.3 % of the global electricity consumption – in some countries and regions, wind energy already contributes 40 % and more. Total installed capacity has increased from 40 GW in 2003 to 93.85 GW in 2007. The following chart gives a country-wise installed capacity of a few countries.

Solar Energy:

Source: International Energy Agency: Solar Heating and Cooling Program-http://www.iea-shc.org/

Sun is an eternal source of energy. However, its potential as a source of energy is only being appreciated now. With an installed capacity of 70 GWth (as in 2001), solar thermal is one of the leading sources of renewable energy worldwide, and its potential is much higher. A number of solar thermal and solar photovoltaic power plants are coming up, which will generate hundreds of MWs of power. Solar energy can also be used in a variety of other applications such as cooking, heating, ventilation and air-conditioning, water heating, desalination and disinfection, lighting, etc. Presently the largest solar thermal power plant is in the Mojave Desert, California, with an installed capacity of 354 MW. The largest photovoltaic plant, however, is much smaller, having a capacity of 23 MW. Plants in excess of 500 MW capacities are under construction, thus promising solar energy a bright future.

Source: REN21-http://ren21.net/

Grid-connected solar photovoltaic (PV) continues to be the fastest-growing power generation technology in the world, with 50 percent annual increases in cumulative installed capacity in both 2006 and 2007, to an estimated 7.8 GW by the end of 2007. This capacity translates into an estimated 1.5 million homes with rooftop solar PV feeding into the grid worldwide.

Hydropower:

Hydropower includes all kinds of energy sources harnessing the power of running water. They are:

1) Hydroelectricity

2) Tidal power

3) Wave power

4) Micro-hydel power-both dammed and run-of-river

5) Ocean thermal Energy Conversion

Hydroelectricity is the most widely used form of renewable energy. Hydroelectricity supplies about 715,000 MW or about 19% of world electricity generation, accounting for over 63% of the total electricity from renewable sources in 2005.

The world’s first commercial wave farm is planned for Portugal, at the Aguçadora Wave Park near Povoa de Varzim. The wave farm will use three Pelamis P-750 machines with a capacity of 2.25 megawatts, enough to meet the average electricity demand of more than 1,500 Portuguese households.

Ocean thermal energy conversion (OTEC) is a method for generating electricity, which uses the temperature difference that exists between deep and shallow waters to run a heat engine. OTEC projects on the drawing board include a small plant for the U.S. Navy base on the British-administered island of Diego Garcia in the Indian Ocean. OCEES International, Inc. is working with the U.S. Navy on a design for a proposed 13 MW OTEC plant, which would replace the current power plant running diesel generators. The OTEC plant would also provide 1.25 MGD of potable water to the base. A private U.S. company also has proposed building at 10 MW OTEC plant on Guam.

During 2005, small hydro installations grew by 8% to raise the total world small hydro capacity to 66 GW. Over 50% of this was in China (with 38.5 GW), followed by Japan (3.5 GW) and the United States (3 GW). China plans to electrify a further 10,000 villages by 2010 under their China Village Electrification Program using renewable energy, including further investments in small hydro and photovoltaic.

Geothermal power:

Geothermal provides almost 10 GW of power capacity, growing at roughly 2–3 percent per year. Most of this is in Italy, Indonesia, Japan, Mexico, New Zealand, the Philippines, and the United States, with additional capacity in several other countries. Iceland gets a quarter of all its power from geothermal.

Bioenergy:

Source: REN21-http://www.ren21.net/

An estimated 45 GW of biomass power capacity existed in 2006. The United Kingdom has seen recent growth in “co-firing” (burning small shares of biomass in coal-fired power plants). The use of biomass for district heating and combined heat-and-power (CHP) has been expanding in Austria, Denmark, Finland, Sweden, and the Baltic countries, and now provides substantial shares (5–50 percent) of district heating fuel. Bioenergy also includes energy from ethanol and biodiesel. Production of fuel ethanol for vehicles reached 39 billion liters in 2006, an 18 percent increase from 2005. Biodiesel production jumped 50 percent in 2006, to over 6 billion liters globally.

Solar Concentrator Innovation:

The goal of any solar concentrator is to concentrate the light that falls on a large area to a smaller one. Usually concentrators are large mirrors, lenses or other devices. The focused light increases the electrical power obtained from each solar cell “by a factor of over 40”.

MIT engineers report a new approach to harnessing the sun’s energy that windows can be used for this purpose. As a result, rather than covering a roof with expensive solar cells (the semiconductor devices that transform sunlight into electricity), the cells only need to be around the edges of a flat glass panel .Light is collected over a large area [like a window] and gathered, or concentrated, at the edges. Solar concentrators, in use today, track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain. Also solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighboring concentrators.

Mechanism of the Window Concentrator

The MIT solar concentrator involves a mixture of two or more dyes that is essentially painted onto a pane of glass or plastic. The dyes work together to absorb light across a range of wavelengths, which is then re-emitted at a different wavelength and transported across the pane to waiting solar cells at the edges. A mixture of dyes in specific ratios, applied only to the surface of the glass, that allows some level of control over light absorption and emission. The mixture was made so that light can travel a much longer distance and light transport losses can be reduced, resulting in a tenfold increase in the amount of power converted by the solar cells.

Economic Feasibility:

Because the system is simple to manufacture, the team believes that it could be implemented within three years–even added onto existing solar-panel systems to increase their efficiency by 50 percent for minimal additional cost. That, in turn, would substantially reduce the cost of solar electricity.

Continuous Improvement in Solar Cell Efficiency:

Most of today’s solar cells are between 12% and 18% efficient. Some of the ones used to power satellites are around 28% efficient. In 1954, 4% efficiency was state of the art. In 2006 Boeing-Spectrolab managed to create a solar cell with 40.7% sunlight-to-energy conversion efficiency. Recently, just after two years scientists at the US Department of Energy’s National Renewable Energy Laboratory have developed solar cells with efficiency 40.8%. These cells are basically multi-junction solar cells and composed of several layers, with each slice capturing only a portion of the solar spectrum; this method of optical concentration is what has allowed cells to surpass the 12% to 18% efficiency barrier faced by most traditional modules.

References:

(1) http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=196602149

(2) http://web.mit.edu/newsoffice/2008/solarcells-0710.html

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