Wednesday, January 26, 2011


Looks like we may still have to bear with NRM. If that is so, we must have a more active civil society and we should give them the direction since they are off course but are not ready yet to get out of office. There is a lot of sunlight energy, but people are talking with no innovations worth mentioning. We want people to reduce the rate at which they are exploiting the forests and environment in a non sustainable way, but we are behind schedule in that effort yet elsewhere a lot has been done to see these innovations. Cases in point:

Bright LED Lamp
Posted by Abigail Kehr 8 Jan 2011 - 16:57

The 12LED lamp provide many hours of bright light. The lamp comes with a rechargeable battery that can be charged with the included solar panel. The rechargeable battery in the lamp will last up to 2 years depending on how the lamp is used. Replacement batteries and replacement LEDs are available. The battery will fully recharge in 5.5 hours, if the panel is placed in direct sunlight. Please keep the lamp inside when charging.

Kit contents:
- 12LED lamp
- 1.0W Solar Panel

Product Specifications:
Battery: 900mAh NiCd (2 year*), 1200mAh NiMd (4 year^)
Solar Panel: 1.0W Polycrystaline
Panel Wire: 4 meters
Battery Protection: Overcharge and Overdischarge
Runtime HIGH Setting: 4 hours
Runtime on MEDIUM Setting: 7 hours
Runtime LOW Setting: 50 hours
Charging Time: 6hrs Solar or 3hrs AC
AC Charging: an AC charger is also available
*400-500 cycles, 1 cycle = 1.5 nights @4hours/night = 2 years (may vary depending on user behavior)
^800-1000 cycles, 1 cycle = 3 nights @ 4hours/night = 4 years (may vary depending on user behavior)

The three most common types of solar cookers are box cookers, curved concentrators (parabolics) and panel cookers. Hundreds — if not thousands — of variations on these basic types exist. Additionally, several large-scale solar cooking systems have been developed to meet the needs of institutions worldwide.

Box cookers cook at moderate to high temperatures and often accommodate multiple pots. Worldwide, they are the most widespread. There are several hundred thousand in India alone.
Overview of box-type designs
Box-type cookers are another group of old but popular type of solar cookers. The very first design of box-type cooker was probably that of Nicholas-de-Saussure (1740-1799). It was simply an insulated box with glazing; this design forms the basis of all the present designs of box-type cookers including Richard Wareham’s (1995) Sunstove.
The box cookers presented here are classified on the basis of presence or absence of reflectors, i.e., those without or with mirrors boosters. The cookers of the second category are further divided into five groups.


By Mark Aalfs, Solar Cookers International -

People use solar cookers primarily to cook food and pasteurize water, although additional uses are continually being developed. Numerous factors including access to materials, availability of traditional cooking fuels, climate, food preferences, cultural factors, and technical capabilities, affect people's approach to solar cooking.
With an understanding of basic principles of solar energy and access to simple materials such as cardboard, aluminum foil, and glass, one can build an effective solar cooking device. This paper outlines the basic principles of solar box cooker design and identifies a broad range of potentially useful construction materials.
These principles are presented in general terms so that they are applicable to a wide variety of design problems. Whether the need is to cook food, pasteurize water, or dry fish or grain; the basic principles of solar, heat transfer, and materials apply. We look forward to the application of a wide variety of materials and techniques as people make direct use of the sun's energy.

The following are the general concepts relevant to the design or modification of a solar box cooker:
The basic purpose of a solar box cooker is to heat things up - cook food, purify water, and sterilize instruments - to mention a few.

A solar box cooks because the interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. It turns to heat energy when it is absorbed by the dark absorber plate and cooking pots. This heat input causes the temperature inside of the solar box cooker to rise until the heat loss of the cooker is equal to the solar heat gain. Temperatures sufficient for cooking food and pasteurizing water are easily achieved.
Given two boxes that have the same heat retention capabilities, the one that has more gain, from stronger sunlight or additional sunlight via a reflector, will be hotter inside.
Given two boxes that have equal heat gain, the one that has more heat retention capabilities - better insulated walls, bottom, and top - will reach a higher interior temperature.

The following heating principles will be considered first:

Greenhouse effectEdit Greenhouse effect sectionEdit

This effect results in the heating of enclosed spaces into which the sun shines through a transparent material such as glass or plastic. Visible light easily passes through the glass and is absorbed and reflected by materials within the enclosed space.
The light energy that is absorbed by dark pots and the dark absorber plate underneath the pots is converted into longer wavelength heat energy and radiates from the interior materials. Most of this radiant energy, because it is of a longer wavelength, cannot pass back out through the glass and is therefore trapped within the enclosed space. The reflected light is either absorbed by other materials within the space or, because it doesn't change wavelength, passes back out through the glass.
Critical to solar cooker performance, the heat that is collected by the dark metal absorber plate and pots is conducted through those materials to heat and cook the food.

The more directly the glass faces the sun, the greater the solar heat gain. Although the glass is the same size on box 1 and box 2, more sun shines through the glass on box 2 because it faces the sun more directly. Note that box 2 also has more wall area through which to lose heat.

Single or multiple reflectors bounce additional sunlight through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures.
The Second Law of Thermodynamics states that heat always travels from high to low energy. Heat within a solar box cooker is lost in three fundamental ways: Conduction, Radiation, and Convection.

The handle of a metal pan on a stove or fire becomes hot through the transfer of heat from the fire through the materials of the pan, to the materials of the handle. In the same way, heat within a solar box is lost when it travels through the molecules of tin foil, glass, cardboard, air, and insulation, to the air outside of the box.
The solar heated absorber plate conducts heat to the bottoms of the pots. To prevent loss of this heat via conduction through the bottom of the cooker, the absorber plate is raised from the bottom using small insulating spacers as in figure 6.

Things that are warm or hot -- fires, stoves, or pots and food within a solar box cooker -- give off heat waves, or radiate heat to their surroundings. These heat waves are radiated from warm objects through air or space. Most of the radiant heat given off by the warm pots within a solar box is reflected from the foil and glass back to the pots and bottom tray. Although the transparent glazings do trap most of the radiant heat, some does escape directly through the glazing. Glass traps radiant heat better than most plastics.

Molecules of air move in and out of the box through cracks. They convect. Heated air molecules within a solar box escape, primarily through the cracks around the top lid, a side "oven door" opening, or construction imperfections. Cooler air from outside the box also enters through these openings.

As the density and weight of the materials within the insulated shell of a solar box cooker increase, the capacity of the box to hold heat increases. The interior of a box including heavy materials such as rocks, bricks, heavy pans, water, or heavy foods will take longer to heat up because of this additional heat storage capacity. The incoming energy is stored as heat in these heavy materials, slowing down the heating of the air in the box.
These dense materials, charged with heat, will radiate that heat within the box, keeping it warm for a longer period at the day's end.

There are three types of materials that are typically used in the construction of solar box cookers. A property that must be considered in the selection of materials is moisture resistance.

* Structural material
* Insulation
* Transparent material
* Moisture resistance

Structural materials are necessary so that the box will have and retain a given shape and form, and be durable over time.
Structural materials include cardboard, wood, plywood, masonite, bamboo, metal, cement, bricks, stone, glass, fiberglass, woven reeds, rattan, plastic, papier mache, clay, rammed earth, metals, tree bark, cloth stiffened with glue or other material.
Many materials that perform well structurally are too dense to be good insulators. To provide both structural integrity and good insulation qualities, it is usually necessary to use separate structural and insulating materials.
In order for the box to reach interior temperatures high enough for cooking, the walls and the bottom of the box must have good insulation (heat retention) value. Good insulating materials include: aluminum foil (radiant reflector), feathers (down feathers are best), spun fiberglass, rockwool, cellulose, rice hulls, wool, straw, and crumpled newspaper.
When building a solar cooker, it is important that the insulation materials surround the interior cooking cavity of the solar box on all sides except for the glazed side -- usually the top. Insulating materials should be installed so that they allow minimal conduction of heat from the inner box structural materials to the outer box structural materials. The lower the box heat loss, the higher the cooking temperatures.
At least one surface of the box must be transparent and face the sun to provide for heating via the "greenhouse effect." The most common glazing materials are glass and high temperature plastics such as oven roasting bags. Double glazing using either glass or plastic affects both the heat gain and the heat loss. Depending on the material used, the solar transmittance - heat gain - may be reduced by 5-15%. However, because the heat loss through the glass or plastic is cut in half, the overall solar box performance is increased.
Most foods that are cooked in a solar box cooker contain moisture. When water or food is heated in the solar box, a vapor pressure is created, driving the moisture from the inside to the outside of the box. There are several ways that this moisture can travel. It can escape directly through box gaps and cracks or be forced into the box walls and bottom if there is no moisture barrier. If a box is designed with high quality seals and moisture barriers, the water vapor may be retained inside the cooking chamber. In the design of most solar box cookers, it is important that the inner-most surface of the cooker be a good vapor barrier. This barrier will prevent water damage to the insulation and structural materials of the cooker by slowing the migration of water vapor into the walls and bottom of the cooker.
Box cookers without reflectors

Curved concentrator cookers, or "parabolics," cook fast at high temperatures, but require frequent adjustment and supervision for safe operation. Several hundred thousand exist, mainly in China. They are especially useful for large-scale institutional cooking.


Panel cookers incorporate elements of box and curved concentrator cookers. They are simple and relatively inexpensive to buy or produce. Solar Cookers International's "CooKit" is the most widely used combination cooker.

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