Build your own Solar Stove

The National Commission for the Saving of Water is taking steps to transform the market for the massive use of solar water-heating systems (SWS) in the domestic sector. The objective is double: first, to save and promote the improvement of renewable energies, using solar radiation instead of LP gas (LPG) or natural gas (NG).

A research made in Mexico indicates the main problems for massive use of SWS in Mexico are: the high cost of the initial investment, which is translated in the necessity of suitable schemes of commercialization and financing; the lack of norms and procedures to guarantee the quality in its installation and operation; as well as the absence of diffusion strategies, promotion and spreading of the technology.

Technologic Innovation

Types of Water Heaters

Based on the type of energy that uses, water heaters for houses can be classified in: those that consume gas exclusively, those that use electricity and those that take advantage of the solar energy. In the case of the gas heaters, three models exist: storage, instantaneous and fast recovery.

The storage water heaters, also known as deposit, warm water contained in a tank to be used when it is required. Whenever this one is extracted, it is replaced by water at an average temperature, that is warmed again.

Instantaneous water systems have a coil from which water is heated to a uniform temperature when the user opens the correspondent tap.

Water heaters of fast recovery are a combination of both previous: by a small deposit, they maintain the water to a uniform temperature, and when they ignite, they keep on continuously heating water through one or more heat exchangers.

There are 2 types: storage and instantaneous.

A SWS is a system that captures sunlight to heat water, and they have a deposit to storage warm water.
Two types of Solar Heater exist, based on the used capturing material: the flat solar collectors and those of evacuated tubes.

The flat solar collectors have as capturing element a copper plate. By his characteristics, they are also called of low temperature, because they only reach between 30 and 60 °C.

Collectors of evacuation tubes are used as solar collector an adjustment of two concentric crystal tubes, with emptiness between both, where they located one in the interior. This equipment, also called of high temperature, can reach up to 80°C. On the other hand, from the operational point of view, solar heaters are classified in: Thermalsiphonics, with integrated endorsement and heat exchanger.

Thermalsiphonic system more commonly is used in the residential sector, and must its name to the phenomenon that causes that water between the capturing element and the deposit flows, also called thermo tanks.

Systems with heat exchanger, also known as of forced circulation, do not store hot water in the thermo tank, they take heat from a fluid (water or another liquid) that circulates in the solar collector they take and it to a tank, which, as well, transfers the heat to the sanitary water. In some cases, these thermo tank counts on an electrical resistance, which warms up the water when the temperature descends to a predetermined level. The call system of integrated endorsement is identical to the thermalsiphonic, unless in the thermo tank is an electrical resistance that it warms up the water under a control system or when asks for it to the user.

Which solar heater to choose?

Selection of the most suitable equipment depends, mainly, of the installation place. The heaters of storage and fast recovery are designed to work at outdoors, whereas those “instantaneous” must settle in interiors, closely together of the place where hot water is used. Also, these last ones are more efficient (they turn into heat between 85 and 90% of the fuel) and consume less gas, because they only ignite when then demand water.

Nevertheless, it must be indicated that technology of the gas heaters is changing: efficiency of storage has improved from 74 up to 80%.

In the case of an instant heater, most recent models no longer use “pilot”, they ignite by electronic means, which represents a very important saving in the gas consumption, because a “pilot” consumes about 0,57 liters of gas LP per day.

Talking about the selection of a good Solar Heater, the user must keep in mind:

i) Required volume of water to the day (in liters).
ii) Wished temperature (for example, 60°C).

More considerations are indicated ahead to define the size of the equipment, based on the final uses.
Furthermore, it is precise to consider the change of the concept of exclusively use a gas heater of domestic use. The tendency now is to have a hybrid system, which consists of installing a gas heater and a heater solar in series or combined.

The reason is simple: to use gas is only equivalent to waste a non-renewable resource that, at the end, is more expensive; but to rely only on the solar heater does not assure to us to have hot water when we use more than the required amount in normal days. In addition, changing conditions of the time usually limit the solar resource and would be too expensive to count on a system for extreme situations.

How much water is consumed in a household?

To choose a heater, the first step is to define how much hot water is required, both in a simultaneous way and through the day.

When talking about gas heaters, selection becomes really simple: it will be in function of the number of services, a service is to spend current water simultaneously.

Manuals offer the next definition:

- 1 service = 1 bath
-½ Service = 1 sink (dish washer)

In a ratio with the water consumption per service, these examples may be considered as averages:

• Shower: 9 liters/minute (normed)
• Kitchen: 4.5 liters/minute
• Sink: 6.4 liters/minute

When it comes to the solar heater calculation is much more difficult. The user must calculate, as precise as possible, the amount of hot water to consume per day.

There is always a “bargain” between the user and the company (or the person) who is installing the system, because the first wants the smaller system, saying his consumption is low, while the second insists on the appropriate heater according to your real water consumption habits.

Vital aspects to consider are the users’ habits, in function of their ages and climatic conditions. Surveys identify these as important matters:

- Older people take faster baths.
- Younger people take a bath:
+ until they ran out of water, (spending 2 or 3 times more than an adult).
+ twice a day (depending on the time of the year and regular activities).
- In extreme climates:
+ In winter people heat from their bath.
+ In summer, they use warm water.

Talking about water consumption by final users, there are researches that shows how and how much hot water is used in households:

• Characteristics of a shower
– at least 10 minutes
– 65% hot water, 35% cold water

A practical recommendation, is to consider 50 liters per person/day and 25 liters for the washing machine, dish washer, etc. Requirements of hot water must be evaluated with distributors in function to the consumption and the uses of solar heater.