The thermodynamics of an oxidation/reduction reaction is the same as that found for any other chemical reaction. The only difference is that the free energies are given in terms of electrical potential with respect to an arbitrary reference state defined by the oxidation/reduction potential of hydrogen. The electrical potental is a measure of the relative energy of an electron on a molecule/atom.

An example of a normal type of redox reaction is:

Fe3+ + Cu+ <-> Fe2+ + Cu2+
This reaction can be broken down into two half reactions:

Fe3+ + e- <-> Fe2+
Cu+ <-> Cu2+ + e-
In this case the iron is reduced from the 3+ state to the 2+ state. In biological oxidations and reductions the electrons are often carried by protons (i.e. the hydrogen atom or hydride, H-). Thus, a general rule for oxidation-reduction is:
Loss of electrons or hydrogen = oxidation
Gain of electrons or hydrogen = reduction

For example, in a battery made of a solution of Fe2+/Fe3+ in one cell and hydrogen in the other, the following reactions will occur (if the concentration of each species is 1 M). The reaction as written is spontaneous. This indicates that the electron is more stable on the Fe2+ than on the Fe3+. The reaction can be written as a sum of two half-reactions:

The initial voltage across the cells will be +0.77 volts. As the iron becomes reduced (and the hydrogen oxidized), the voltage will drop, becoming zero at equilibrium. The net free energy that can be obtained from this reaction is:

The negative sign indicates that the reaction is spontaneous as written.
n is the number of electrons involved in the transfer (one in this case).
F is the Faraday constant = 96,494 C/mol = 96,494 joules/volt-mol.
∂E is the difference between the redox potential of each half-reaction.
The redox potential is the voltage obtained for a redox reaction relative to that of hydrogen, all reactants being at standard state (1 M). The standard half reaction potential (Eao) is measured relative to reduction of hydrogen at pH 0, 25°C and 1 atm H2 gas (i.e. Eao for this reaction is zero).
The free energy change of any reaction is given by the following.

This important equation is called the Nernst equation; it gives the electrical potential that can be obtained from an oxidation-reduction reaction. As with ∂Go, the standard redox potential (∂Eo) is the potential that would be observed when the concentration of the products and reactants are 1 M.

For any two redox pairs the overall redox potential is calculated according to the following (Eodonor = 0 for the reduction of hydrogen).

The second law of thermodynamics states that the universe (i.e. all systems) tend to the greatest degree of randomization. This concept is defined by the term entropy, S.

S = klnW

where k = Boltzmann constant (the gas constant, R, divided by Avagadros\' number) and W = the number of substrates. For an isothermal reversible reaction the change in entropy can be reduced to the term:

ΔS = ΔH/T

Whereas, enthalpy is a term whose value is largely dependent upon electronic internal energies, entropy values are dependent upon translational, vibrational and rotational internal energies. Entropy also differs from enthalpy in that the values of enthalpy that indicate favored reactions are negative and the values of entropy are positive. Together the terms enthalpy and entropy demonstrate that a system tends toward the highest entropy and the lowest enthalpy.

In order to effectively evaluate the course (spontaneity or lack there of) of a reaction and taking into account both the first and second laws of thermodynamics, Josiah Gibbs defined the term, free energy which is defined as:

ΔG = ΔH – TΔS

Free energy is a valuable concept because it allows one to determine whether a reaction will proceed and allows one to calculate the equilibrium constant of the reaction which defines the extent to which a reaction can proceed. The discussion above indicated that a decrease in energy, a negative ΔH, and an increase in entropy, a positive ΔS, are indicative of favorable reactions. These terms would, therefore, make ΔG a negative value. Reactions with negative ΔG values are termed exergonic and those with positive ΔG values endergonic. However, when a system is at equilibrium:

ΔG = 0

Gibbs\' free energy calculations allows one to determine whether a given reaction will be thermodynamically favorable. The sign of ΔG states that a reaction as written or its reverse process is the favorable step. If ΔG is negative then the forward reaction is favored and visa versa for ΔG values that are calculated to be positive.

In an idealized irreversible reaction such as one done by expanding an ideal gas against zero pressure, no work will be done by or on the system so the:

w = 0

In the case of an ideal gas (whose molecules do not interact) there will be no change in internal energy either so:

ΔE = 0

since ΔE = q – w, in this irreversible reaction q = 0 also.

In a reversible reaction involving an ideal gas, ΔE still will equal zero, however, the pressure will be changing continuously and work (w) is a function of P, work done must be determined over the entire course of the reaction. This result in the following mathematical reduction:

w = RTln[V2/V1]

Since in this situation ΔE = 0, q = w. This demonstrates that some of the heat of the surroundings has to be absorbed by the system in order to perform the work of changing the system volume.

Reversible reactions differ from irreversible in that the former always proceeds infinitely slowly through a series of intermediate steps in which the system is always in the equilibrium state. Whereas, in the irreversible reaction no equilibrium states are encountered. Irreversible reactions are also spontaneous or favorable processes. Thermodynamic calculations do not give information as to the rates of reaction only whether they are favorable or not.

The total energy of the universe does not change. This does not mean that the form of the energy cannot change. Indeed, chemical energies of a molecule can be converted to thermal, electrical or mechanical energies.
The internal energy of a system can change only by work or heat exchanges. From this the change in the free energy of a system can be shown by the following equation:
ΔE = q – w
When q is negative heat has flowed from the system and when q is positive heat has been absorbed by the system. Conversely when w is negative work has been done on the system by the surrounding and when positive, work has been done by the system on the surroundings.

In a reaction carried out at constant volume no work will be done on or by the system, only heat will be transferred from the system to the surroundings. The end result is that:
ΔE = q

When the same reaction is performed at constant pressure the reaction vessel will do work on the surroundings. In this case:
ΔE = q – w
where
w = PΔV

When the initial and final temperatures are essentially equal (e.g. in the case of biological systems):

ΔV = Δn[RT/P]

therefore,
w = ΔnRT
By rearrangement of the above equations one can calculate the amount of heat released under constant pressure:
q = ΔE + w = ΔE + PΔV = ΔE + ΔnRT
In this last equation, Δn is the change in moles of gas per mole of substance oxidized (or reacted), R is the gas constant and T is absolute temperature.

Enthalpy
Since all biological reactions take place at constant pressure and temperature the state function of reactions defined to account for the heat evolved (or absorbed) by a system is enthalpy given the symbol, H

The changes in enthalpy are related to changes in free energy by the following equation:
ΔH = ΔE + PΔV

The equation is written in this form because we are addressing the constant pressure situation. In the biological setting most all reaction occur in a large excess of fluid, therefore, essentially no gases are formed during the course of the reaction. This means that the value ΔV, is extremely small and thus the product PΔV is very small as well. The values ΔE and ΔH are very nearly equivalent in biological reactions.

Stated above was the fact that state functions, like ΔH and ΔE, do not depend on the path taken during a reaction. These functions pertain only to the differences between the initial and final states of a reaction. However, heat (q) and work (w) are not state functions and their values are affects by the pathway taken.

1.1 The Sun
The sun is the closest star to the earth, and it is possible to understand how such an enormous sphere of gases, mainly hydrogen and helium at very high temperatures; inside, reaches temperatures up to 40 million Kelvin degrees, and on their surface reach 6000 Kelvin degrees approximately. The origin of the enormous radiant amount of energy that comes from the sun are the thermonuclear processes that happen inside, nuclear fusion processes where hydrogen, as basic fuel is transformed in helium and in this process gigantic quantities of radiant energy are generated. The sun has a diameter of 1.39 x 109 m and is located at 1.49 x 10 11 m from the earth, the earth has a diameter of 6.37 x 106 m. 

1.2 Solar Energy 
As it is known, solar energy coming to the earth comes from the sun. Solar energy is free and inexhaustible. It supports all life on earth and it is the basis for almost every form of energy we use. The amount of energy from the sun that falls on the earth is enormous. All the energy stored in the earth\'s reserves of coal, oil, and natural gas is matched by the energy from 20 days of sunshine. Outside the earth\'s atmosphere, the sun\'s energy contains about 1,300 watts per square meter. About one-third of this light is reflected back into space, and some is absorbed by the atmosphere (in part causing winds to blow).

By the time it reaches the earth\'s surface, the energy in sunlight has fallen to about 1,000 watts per square meter, at noon on a cloudless day. Averaged over the entire surface of the earth, 24 hours per day for a year, each square meter collects about the energy equivalent of a barrel of oil. So each day, on average, a square meter collects 4.2 kilowatt-hours of energy. This figure varies by location and by weather patterns. Deserts, with very dry air and little cloud cover, receive the most sun, more than 6.0 kilowatt-hours per day per square meter. 

Solar Energy has been and is the most important energy source in the earth, it has allowed the evolving and evolution of life on it. For the human being, solar energy, has played an essential roll.

2 Use of solar energy in Building Planning and Design

Buildings designed for passive solar and day-lighting use features such as large south-facing windows and materials that absorb and slowly release the sun\'s heat. No mechanical means are employed in passive solar heating. Passive solar designs can reduce heating bills as much as 50 percent. These solar designs can also include natural ventilation for cooling.

2.1 Passive Solar Design
Passive solar designs are simple and of greater reliability, lower costs, and longer system life times. Passive solar heating and cooling of buildings seem to be cost effective, when appropriate techniques suited to the regional and micro climate of the place and site are chosen. Climate conscious and energy efficient designs should supplement the passive solar designs. Passive systems when effectively designed, would perform effortlessly and quietly, without mechanical and electrical assistance. They do not require special construction skills and much can be accomplished using locally available common building and insulating material. The additional financial investment may range from nil to 10 percent. Passive solar systems save fuel bills for the house owner and the nation.

Passive solar design uses sunshine to heat and light homes. It is usually part of the design of the building itself, using certain materials and placement of windows or skylights. A successful passive solar building needs to be very well insulated in order to make best use of the sun\'s energy. The result is a quiet and comfortable space, free of drafts and cold spots. Passive solar design can also achieve summer cooling and ventilating by making use of convective air currents which are created by the natural tendency of hot air to rise. 

In the winter when heating is required, the sun is low in the sky, which allows the heat to penetrate into windows on the south face of a structure. In the summer, south-facing windows can be shaded by an overhanging roof or awning to keep out the high hot summer sun. Because much of a building\'s heat is lost through its windows, the majority of windows in a passive solar building are located on the south wall. 

2.1.1 Orientation
In order for passive solar systems to work effectively, care must be take to ensure that the building is oriented to take advantage of year-round energy savings. The ideal orientation for solar glazing is within 5 degrees of true south. This orientation will provide maximum performance. Glazing oriented to within 15 degrees of true south will perform almost as well, and orientations up to 30 degrees off—although less effective—will still provide a substantial level of solar contribution. The warmer the climate, the more east and west-facing glass will tend to cause overheating problems. In general, southeast orientations present less of a problem than southwest. In the ideal situation, the house should be oriented east-west and so have its longest wall facing south. But as a practical matter, if the house\'s short side has good southern exposure it will usually accommodate sufficient glazing for an effective passive solar system, provided that the heat can be transferred to the northern zones of the house. 

2.1.2 Direct Gain
The most common passive solar system is called direct gain. Direct gain refers to the sunlight that enters a building through windows, warming the interior space. During the sunlight hours, this heat can be stored in thermal mass incorporated into floors or interior walls made of adobe, brick, concrete, stone, or water. The heat held by the thermal mass will continue to radiate into the space after the sun goes down. Designing a direct gain system includes calculating how much window area and how much thermal mass are required to provide the desired quantity of heat for the space. In general, total direct gain glass area should be at least 7 percent, but not exceed 12 percent of the house\'s floor area. Night insulation, such as window shades, quilts or insulating drapes, improves energy efficiency dramatically. 

2.1.3 Solar Control
To minimize the exposure of wall and window areas of the building, to the direct solar radiation by shading them is the basic approach of solar control. By treating the exposed wall and roof surfaces with white, off white, or light colour surface finishes, as much as 60 to 70 percent of solar energy can be rejected. . However shading of. windows is far more important as unshaded. clear window glasses, when exposed to direct solar radiation, transmit more than 85 percent. Once the sunlight hits the window glass, half the cooling battle is lost. South facing windows admit less sun in summer than in winter and it is also easy to shade them with overhangs. East and west window glasses are difficult to shade with overhangs alone. Vertical fins are the best means of shading such glasses. We all know that the sun path varies during the day from season to season and from place to place.

Fortunately solar geometry is well established. Once the latitude of a place is known, the sun angles with respect to horizontal, vertical and sloping surfaces or any orientation can be calculated or graphically determined with solar chart and shadow angle protractor, for any hour of the day and any day of the year. Shading effects of trees and adjacent buildings can also be determined by solar geometry. Operable internal shades, shutters, and curtains and the use of special glasses such as heat reflecting, heat absorbing and low emissivity types would provide some solar control but. reduce daylight day time heat gain control and also promoting night heat losses,

2.1.4 Daylighting 

Daylighting, the use of natural light (indirect sunlight) to illuminate building interiors, is a well-known strategy for shaving a building\'s electric load at peak demand periods. Daylighting can cut peak loads by as much as two-thirds in new construction and nearly half in major retrofit projects. Daylighting is not only the coolest source of light available to building designers, but potentially the least ex-pensive as well. As energy consumption in commercial buildings is dominated by the need to light and cool interior spaces, reducing the amount of heat-producing electric lighting per square foot reduces cooling loads as well. Architects are becoming increasingly aware that buildings with properly controlled natural lighting are more pleasant spaces to work or learn than conventional buildings. 

South Facing Glass

South facing glass, also called glazing, is a key component of any passive solar system in the northern hemisphere. The system must include enough solar glazing for good performance in winter, but not so much that cooling performance in summer will be compromised. When the solar glazing is tilted, its winter effectiveness as a solar collector increases. However, tilted glazing can cause serious overheating in the summer if it is not shaded very carefully. Ordinary vertical glazing is easier to shade, less likely to overheat, less susceptible to damage and leaking, and so is almost always a better year-round solution. Even in the winter, with the sun low in the sky and reflecting off snow cover, vertical glazing can often offer energy performance just as effective as tilted. 

Thermal Mass

Almost all passive solar systems work in conjunction with thermal mass, or materials with a high capacity for absorbing and storing heat (e.g., brick, concrete masonry, concrete slab, tile, adobe, water). Thermal mass can be incorporated into a building design as floors and interior walls. The sun does not need to hit these surfaces directly to store the heat, nor do these surfaces necessarily need to be a dark color. The thermal storage capabilities of a given material depend on the material\'s thermal conductivity, specific heat and density. Conductivity tends to increase with increasing density; generally, the higher the density of the material, the better. 
Effective materials for floors include painted, colored or acid-etched concrete, brick, quarry tile, and dark ceramic tile. When more mass is required, interior walls or interior masonry fireplaces can be incorporated into the design. Mass walls serve the dual functions of serving as structural elements or fire protection as well as for thermal storage. From an energy standpoint, it would be difficult to add too much thermal mass in a house. But thermal mass has a cost, and so adding too much mass just for thermal storage purposes can be unnecessarily expensive. As with all aspects of solar design planning, it is necessary to achieve a workable balance. 

2.2 Advantages to Solar Heating/Cooling 
• High Reliability
• Low Operating Costs – Sunlight produces the energy, so the fuel is free.
• Once installed, can operate continuously with little upkeep & minimal costs.
• Environmental Benefits -Clean and Silent
• Modularity -Can be constructed to any size based energy requirements.
• Low Construction Costs

Disadvantages to Solar Heating/Cooling
• High Capital Cost Large investments have to be made in buying the equipment. It is estimated that the cost for this kind of electricity @ least doubles the cost of conventional electricity.
• Power to be used 24 hours a day (or at night) would require a storage battery – more investment
• Air pollution and Weather Dependent. Dependency on weather and pollution reduces the productivity of the power plants.

2.3 Old Solar Cooling Technique
There was a technique used or invented by the Romans a long time ago. A natural form of air conditioning / ventilation was used roughly as follows: 
• A trench 6 to 12 feet deep and 100 to 200 yards long was dug leading from the \"house\" in a straight line away from the house. 
• Into this trench a large diameter pipe (these days corrugated drainage pipe 2 or 3 feet diameter) was laid, with holes drilled into the bottom to drain water that condensed inside the pipe. The trench was then covered over. 
• At the far end a 90 degree elbow was attached and more pipe added so that it reached above ground and the end covered with some sort of wire mesh attached to keep out unwanted things such as rodents, etc., and then another elbow could be added at this end to shield against rain. 
• The house end of the pipe entered the house and was the source of incoming air. 
• The key to making this work is to add a convection chimney. 
• The Convection chimney is built such that it\'s inside opening is at a high point inside the building. 
• On the outside, two intersecting sides of the chimney; are painted flat black, and the resulting V formed by the two connecting sides face south. In other words, the V needs to face the mid point between where the sun rises and sets. 
The two other sides must be transparent, Plexiglas or some equivalent. Also, the higher/larger the chimney, the better. 

How it works: 

The sun heats up the chimney causing the air inside to rise, thus drawing air through the cool pipe. The pipe cools the air drawn from the outside to the temperature of the earth at the depth at which it is buried (which is virtually constant year around at this depth). By the way, an interesting note: Even in cold climates where the ground is frozen, the incoming air is only 32F when the air outside may be much colder, we need only heat the air by 38F to bring it to 70F; as opposed to heating outside air of say -15F to 70F we would have to heat the incoming air by 85F - quite a difference in the amount of heating energy we would have to supply by some other means.

In most cases, a moisture absorbing material called a desiccant is used to lower the humidity in the air stream to the point that an evaporative cooler can then cool the air. High temperature liquid collectors are used in this system to conduct heat energy which allows system to work. Usually solar space cooling is used in conjunction with space heating because both require similar components and return on investments will not be maximal if only a solar cooling system is used very costly.

3. FINANCING RENEWABLE ENERGY SOURCES 

Leasing can be used to finance most type of RES and EE equipment over the full range of project sizes ranging from large industrial ones such as heat recovery or cogeneration to small, mass market programmes such as solar lantern, solar water heating system, compact fluorescent lighting (CFLs) or power factor correction capacitor installations. This allows the user of a leased asset (the lessee) to avoid using capital up-front to acquire the asset, and the lease can be so structured that cost savings will be greater than the lease payments, thus generating a positive cash flow to the lessee. India has a well established leasing industry and current tax policies allow 100 per cent, one year accelerated depreciation for RES and EE and renewable energy equipment.