Solar energy for your home

You too can harness the power of sun to run your home ap­pliances. Here we share the options to convert solar radiation into electricity, calculation of the solar power needs of your home and some innovative ideas

By P. Rajesh, N. Pavan Kumar And Syed Zameeruddin

Monday, March 25, 2013:There are two major ways to convert the solar radiation into electricity: photovoltaic cells and concentrated solar power. Photovoltaic cells convert the solar power directly into electricity. You may already be familiar with photovoltaic cells if you have ever used a solar calculator. Photovoltaic cells in calculators are a good idea to get rid of batteries but these can’t be used for higher power needs owing to their high cost. Typically, the power efficiency of photovoltaic cells is 12-24 per cent, which is lower than other energy sources. The high cost of photovoltaic cells prevents their wide usage but they do find application in areas like satellites, which need continuous power without batteries, and small devices (calculators and watches).

Concentrated solar power (CSP) systems concentrate the sun beams to produce heat and then convert the heat energy into electricity by conventional ways. There are several designs for concentrating the sun beams but the method is the same in all designs: There is a reflector which concentrates the beams in a heat collector, also called absorber. A fluid (molten salt, for example) flowing in the receiver stores the heat. The hot fluid flows to the heat engine and the heat energy is converted into electricity.

CSP system designs

Parabolic-trough design. In parabolic design, there are parabolic mirrors collecting the heat on a receiver that is placed on the mirror’s focus. The energy production is as explained above: The hot fluid is flown through pipes and transferred to the heat engine. This is the most practised method of solar such power plants and there are several such power plants fully operational. Parabolic-trough power plants have an efficiency of 20 per cent, but less area usage and cheap design are their key advantages over other systems.

Power tower design. The power tower design is similar to the parabolic design but it uses a large number of flat, mobile mirrors (also called heliostats) that focus the beams on a single receiver (tower). All the energy focused on a single point creates higher temperatures, thus creating electricity with higher efficiencies. There are several such power tower plants in production. Although parabolic-trough design is proven, the higher efficiencies and lower energy cost predictions are creating more investments for power tower.

Dish design. Dish designs use the same principle as the satellite receivers. The dish collects and focuses the energy at a receiver facing the dish. There are several advantages of dish design: These provide the highest solar-to-electric efficiency (31.25 per cent) among all CSP technologies. Also, each unit has its own standalone structure, which makes it fit for smaller applications (unlike tower design that has a high central tower cost). However, the dish design is costlier than other systems because of its compiler design. Also, it requires more maintenance. Nevertheless, high efficiency is promising a future for dish designs also.

Fresnel reflectors design. Fresnel reflector is a new concept similar to parabolic trough design. It uses a set of flat mirrors, which have a common receiver (unlike parabolic design where each mirror has its own receiver).The advantages of fresnel reflectors are the use of flat mirrors, fewer number of receivers and lower energy station area decreasing the cost dramatically. Although the efficiency of fresnel reflectors is lower than parabolic-trough design, low cost per energy unit can make fresnel reflectors quite popular in the future. Currently, these are in developing stage.

Power consumption of home appliances

Before calculating solar power system needs of your house, you need to calculate the electricity consumption of your house. Power consumption of various home appliances is shown in Table I. These equipment work on 230V, 50Hz input voltage.

Let’s assume you want power backup for electrical load of one tubelight, one fan, one TV, one computer and one bulb. These appliances together will consume 325.8W power. If you take a 400W solar inverter, it will give a power backup of up to 320W, considering all the core and copper losses. If you don’t use a computer, the power consumption reduces to only 245.9 watts.

How many solar panels do you need for your home

To calculate solar power needs of your home, proceed as follows:
1. How much power do you use per day? To calculate this, get your last one year’s monthly power bills and calculate your average kilowatt hour (kWh) usage per month. The reason why we have considered the 12-month time period is because our power consumption fluctuates with the seasons. (In case you don’t have all your power bills, simply use the last month’s.) Now take your average monthly usage and divide it by 30 to work out your average daily power consumption. As we get our electricity bill in units, we have to convert units into watts.

Let’s say the electricity consumption of a house is 168 units per month. So daily power usage is 168/30=5.6 units. As 1 kWh=1 unit, the power consumption in watts would be 5.6 kWh.

2. Calculate the total wattage of solar power needed. Before you can work out number of solar panels you may need, you need to find out how many usable hours of sunlight your region gets each day. A simple way to find that out is to have a look at an isolation map. You could just Google to find one.

Now take the average daily kWh calculation and divide that by the number of daily usable sunlight hours, then multiply that by 1.25 (to take into account the wasted energy from wiring, charge controllers, batteries and inverters).

Continuing from our example, our solar panel watt needs equal:
5.6 kWh needed /8 hrs of sunlight x1.25 =0.875 kW or 875 watts a day
If you want to use a 100W solar panel, you will need 875/100=8.75, i.e., 9 pieces of 100W monocrystalline solar panels.

This indicates that your home solar power system must have the capacity to produce a minimum of 875-watt power. (The backup of system is based on battery capacity.)

ZMR—backup analysis for batteries
The battery backup for the required load can be calculated by Z-factor, where Z=0.2222. This Z-factor has been found out from the observations with batteries used for certain loads. As the wattage increases, this ‘Z’ value also increases. Z-factor decreases with aging of the battery.

Multiplying the load with Z-factor factor gives you the value of battery backup required for one hour. Let’s say you have a 90W load. So for one hour, the battery backup is: 90xZ=19.998 Ah =20 Ah. For six hours, the battery backup required is 90x6Z=119.98 Ah=120 Ah.

Limitations of ZMR backup analysis are:
1. The performance of Z-factor decreases with aging effect of batteries.
2. It supports a load upto 5kW.

If you know the current consumption of the load, it is simple to calculate the Ah for the required load by formula: Ah = I×T

where ‘I’ is current consumption of load in amps and ‘T’ is backup time in hours. For example, if I = 10A and T = 3 hours, Ah = 10×3 = 30 Ah.

As you can see from Table II, a 400W inverter with 12V, 120Ah battery is unable to support a computer with UPS. After two minutes, the voltage drops.

When the UPS is removed and a voltage regulator with 230V output placed, the 400W inverter is able to run the computer without any interruption. The supply to the computer without UPS is shown in Table III. It continues for up to six hours, then shuts down.

New ideas
Given below are some new ideas for making use of solar energy:

Solar batteries. Solar batteries are 12V DC batteries with different Ah capacities. These batteries have 12V solar panels on top of them beside, of course, the charging circuitry. You just have to place the battery in the sun for charging and use it after it is completely charged. A heat-insulating material is coated to prevent the environmental heat from entering the battery, thus protecting the battery from damage due to over-heating.

Solar tanks. Just like a water tank on top of the buildings, we can have solar tanks that absorb the sunlight and supply power to the houses for a long time. These can be built in the same way as a solar battery but have to be bigger in size.

Nano solar cells. As the power consumption increases, the required wattage of the solar panel also increases, requiring an increase in its size. Nanotechnology allows reduction in size of the solar cells while increasing their efficiency.

Solar nets. With computers running on solar energy, there would be no need to pay electricity bill for the computer usage. A package of computer with the solar panel, solar converter and battery would make this possible.

Solar air-conditioners. A conventional split air-conditioner has the condenser installed outside the house and conditioning unit placed inside a room. If the outer body is surrounded by walls of solar panels, it’s possible to run the air-conditioner with solar energy with the help of a small converter and battery.

Solar power generators. The electric generators work on fuel. Due to the combustion of fuel, a lot of gases including CO2 and CO are released into the atmosphere. By replacing the fuel with solar energy, it is possible to generate electricity without any pollution. For example, consider a 5kVA generator. A motor is used to run the generator in order to produce 5kVA supply. The energy required to rotate this motor can come from a solar panel, whose DC supply is converted into AC using a solar inverter.

Solar robots. A robot can be made to work using DC supply for all its PCB components including the microcontroller. For this, the robot can be made with DC components and its outer body covered with solar cells. The solar energy will be directly absorbed by the robot; the robot will recharge when in sunlight and use the stored energy.

Solar laptop. Solar panels can be placed on the body of a laptop to charge the battery as well as use the supply. There can also be the option to use electrical supply when sunlight is unavailable because of the monsoon or night time.

P. Rajesh is an M.Tech and assistant professor, N. Pavan Kumar is MA, M.Phil and assistant professor, and Syed Zameeruddin is a third-year B.Tech student

Electronics Bazaar, South Asia’s No.1 Electronics B2B magazine