Solar water heating, an example of solar thermal technology, has become increasingly popular with Kenyan homeowners. Using the sun to heat water is efficient, saves money, and reduces pollution. There are over 65,000 solar water heaters in use in Kenya today. In fact, Nairobi ranks number one in Eastern Africa nation when it comes to using energy from the sun to heat water.
HOW SOLAR WATER HEATING WORKS
Sunlight is absorbed by one or more solar collectors, which transfer the absorbed solar heat to water circulated through the collector(s). The heated water is then stored for use throughout the day and night in a hot water storage tank that is highly insulated for it not to lose heat.
THERE ARE TWO BASIC KINDS OF SOLAR WATER HEATERS:
Active (forced circulation) systems that use pumps to circulate the water, and
passive (thermo siphon) systems that rely on natural convection for water circulation
Both types of systems include solar thermal collectors and storage tanks. The collectors can range in size from 3 ft. x 7 ft. up to 4 ft. x 12 ft. and are often used in groups. The storage tanks are usually larger than conventional electric water heaters and generally hold 80 or 120 gallons. The tanks may be plumbed together to increase the storage capacity. The heating efficiencies of either system are approximately the same. In areas that experience freezing temperatures, a heat exchanger system is often used but in tropical areas like Kenya, they are not needed.
An active system (forced circulation) uses an electrical pump to circulate water between the solar collectors and the storage tank. The pump may be either a 120V AC pump or a DC pump.
An AC pump is plugged into regular house current and relies on temperature sensors and a differential controller to turn the pump on and off. The sensors indicate whether the water in the collector is sufficiently hotter than the water at the bottom of the storage tank. If it is, the differential controller turns the pump on. Water will then circulate through the system with the hot water in the collectors flowing to the storage tank and being replaced by colder water; then the pump will automatically turn off while the water in the collector heats again. The pump will cycle on and off repeatedly during the day, depending on the amount of heat generated by the sun.
A DC pump gets its power from a small photovoltaic (PV) panel usually mounted on the roof next to the solar collectors. At the same time electricity is being generated to power the pump, sunlight is shining on the solar collectors heating water. When the sunlight is bright enough to generate electricity and power the pump, it is usually also hot enough to heat water. This synchronous use of the sunlight ensures that when the water in the collector is hot, electricity is generated to pump the hot water to the storage tank. The more sunlight received, the faster the water heats and the faster the pump works.
A passive system
A passive (thermo siphon) system relies on natural convection to circulate the hot water. Hot water naturally raises so no mechanical pump (AC or DC) or access to electricity for a pump is needed. It is essential, however, that in a passive system the storage tank be higher than the collectors. As long as the sun shines, water in the collector will heat and move slowly upward into the tank with the colder water descending to replace it. As the storage tank is usually mounted horizontally above the collectors on the roof, the added weight is a consideration.
SOLAR WATER HEATING / POOLS
Not only is Kenya an ideal location for solar water heating for household use, it is also an ideal location for solar pool heating. Solar pool heaters generally do not have to heat the water to the temperature used in most households and the type of solar collector which is used for pool heating is usually different than the glass covered collectors used for domestic water heating.
Solar Modules Solar modules provide some of the highest power density available in the market. They range from 12 Watts to 160 Watts and yield higher current output by 10-17% at operating battery voltage. Ideal for battery charging applications and in stand-alone systems such as rural electrification, lighting, telecommunications, water pumping etc. Supplied and installed in Africa for over 20 years our solar modules have withstood the test of time in some of the toughest working conditions providing much needed free electricity to schools, hospitals, aid project, settlements and missions.
Determining Number of Solar Panels Needed
I. First, reduce your load!
The first thing you need to do (if you haven’t already) is look at ways to reduce your household electrical needs. Big wasteful energy consumers are electric water heaters, stoves and heating systems. If possible, these systems should be converted to gas power – anything besides electrical.
Usually the expense of changing out these systems is less than the added expense of having a solar/wind energy system to power these large electrical energy consumers.
Other obvious things you can do to significantly reduce your energy needs is to convert all incandescent lighting to full-spectrum fluorescent lighting. Again, the added expense of these lighting systems is easily compensated by the reduction in the cost of your needed solar/wind energy system.
Next you need to design as system that will meet your electrical needs. For the sake of simplicity, we’re going to go over the basic steps you would take to design a solar electric system for a home that’s occupied all year around (as opposed to a summer cabin, for instance). The difference being that a home that would be occupied all year round needs to be designed to produce enough energy during the low-sunlight times of the year.
II. Determine the number of solar panels you will need
1) Calculate your electrical load. Determine the number of watt*hours your location will use on a daily basis. For an entire home, this will take some busy work. You need to determine:
What appliances you are going to power.
How many Watts does each appliance consume?
On average, how many hours do you use this appliance per day? (Don’t forget to include those things that you use on a weekly or monthly basis, like a vacuum sweeper or a blender).
Calculate your average daily watt*hour usage for your entire home. That is, multiply the watts of the appliance time the average number of hours used per day.
2) Determine the equivalent number of full-sun hours for your location for the month with the least amount of sunlight (typically December or January).
3) Divide your load calculation from step 1 by the number of full-sun hours from step 2. This is will tell you the number of Watts of solar panels you will need to provide you enough electricity in the lowest sun-light months.
4) Compensate for system inefficiencies. Every part of a solar powered system has some inefficiency in it. The rule of thumb is if you are going to use an inverter (to produce AC) your total system inefficiency will be 30%. For systems that will be using the DC voltage directly from the battery bank, the inefficiency factor is 20%. So, to compensate for inefficiencies multiply your answer to step 3 by 1.3 (or 1.2, if there’s no inverter).
This answer is the number of watts of solar panels you will need to provide enough electricity for your loads.
5) Finally, to determine how many solar panels you will need, take your answer from step 4 and divide it by the rated power output (watts) of the solar panel that you have chosen.
Project Design & Technical Department;
Prittworld Properties & Mortgages Limited;
0722 721 525 / 0739 256 892;
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