Solar Cooker Storage

This page has been compiled to address the Solar Cooking Storage issue with simple, inexpensive and effective methods of heat storage. It is based on some very old methods using stones and large sealed chambers that are outlined in The Ancient Solar Premise

Simple Storage

The first method just has a black sheet of metal wrapped tightly around a solid fired house brick; this is aligned with three others and placed in an insulating box. See diagram. The energy throughput can be increased by increasing the number of metal casings and reducing the size of the bricks. This is a slight improvement on just placing black stones in the base of a box cooker.

Simple Brick & Metal Conductor Storage Array

Storage with High Energy Throughput

The second arrangement increases the throughput of energy, both in and out, by increasing the metal contact area. The same metal sheaf is used as above, but there are pipes welded to the interior. The pipes fit tightly into the holes in hollow fired house bricks. The pipes are cut along one edge to allow for expansion without cracking the bricks.

Brick Storage with Metal plate & pipe conductor enhancements

Usage

Both arrays are used in the same way. Prior to cooking the brick arrays are left in the base of the solar cooker. As the sun heats the metal surface, the energy is transferred to the bricks. This allows the bricks to heat up and store the sun’s energy. If the array is exposed to the air, an insulating lid needs to be put on the metal surface when the sun disappears. This is also beneficial within enclosed box style cookers as an extra insulation measure.

Cooking can be carried out in the normal fashion by placing a pot or tray of food onto the surface whilst exposed to the sun. The heat within the stones allows the cooking to continue even if the sun is obscured.

Alternatively, cooking can be carried out purely with the heat stored in the bricks. At sunset, the food is placed in the box cooker and the lid is closed. This allows the food to slow cook with the heat from the bricks.

With this second approach, there is a clear relation between the amount of food that can be cooked and the mass of the bricks. The bricks should weigh more than the food and be heated to as high a temperature as possible. This is achieved with large reflector panels.

In both cases, the speed of cooking is improved because the heat is being directed at the base of the food. Convection currents take it upwards guaranteeing the whole is at temperature. The colors of the foods are also incidental in this approach since it is the stones and the metal conductor that deliver the heat.

Materials

Steel sheets were used in tests for the first arrangement and proved good enough conductors. Clearly copper would be a much better conductor of heat though significantly more expensive. Iron may be a good compromise.

Bricks were used in all of the tests. The principle is based on storing the sun’s energy in a variety of dark stones such as granite, basalt and obsidian. A variety of materials can be used in the arrangements. Natural stones are the most durable, store energy very well and do not even require the metal sheaf if they are black. However, because of the expense in some regions, fired house bricks prove a reasonable substitute. They can be used with paints, but the throughput is limited by the contact area. The designs above double or triple the surfaces through which the heat can be transferred.

There are composite bricks designed specifically for storing heat used in electric storage heaters. These may prove the best material for the task.

Reservations

There are several issues with this style of solar heat storage.

The primary one is that the stones have to be preheated before cooking occurs. This means there are initiation times  proportionate to the mass of stone and the temperature required. However, this is also the source of the utility, since the cooking can continue at night or whilst there is no sun. This removes the primary temporal restriction on solar cooking,

The brick heat capacities vary greatly from place to place. This is because of the different clays used and the variety of firing temperatures. This is also true of the natural stones, which form with different compositions and conditions. Each region requires the stones to be tested in practice to realize assured guidelines.

The insulator used in tests was a standard polystyrene icebox with layers of aluminum foil and cardboard protection. This proved an excellent insulator. It retained the heat within the stones overnight. The problem was the degradation of the polystyrene over time because of the heat. This reduces both the storage capacity and the time it can be stored. The exact figures for this area are a constantly moving target.