Solar Electric vs Solar Thermal

Solar energy has multiple applications and it is not necessarily converted into electricity in every case. For e.g. a solar cooker uses heat from sun to cook food, a solar water heater again uses heat from sun to heat water. These are solar thermal applications where heat from the sun is collected and utilized or transferred. No electricity is generated in this case.

Another example is the innovative day-lighting solution we discussed in our previous article (link), which does not convert solar energy but just innovatively moves light from sun using reflective surfaces.

None of the above applications use photovoltaic cells (or PV in short) and are highly efficient systems (efficient in utilizing energy from sun) and are cheap as compared to a photovoltaic application. A PV is required when solar energy is used to generate electricity. PVs are made of silicon and available in 2 types: 1) mono-crystalline 2) poly-crystalline. As per their name, they convert Photo (or light) into Volt (or electricity). Their efficiency in utilizing sun’s energy is much less as compared to thermal or photo applications as explained above. The difference between mono-crystalline and poly-crystalline is that mono-crystalline is made of single silicon crystal whereas multi-crystalline PV is made up of multiple crystals. A mono-crystalline is more efficient in converting solar energy into electricity per sq meter area than a multi-crystalline PV. Thus the space required for the same amount of wattage is less in mono-crystalline PV panel and hence it is costlier than a multi-crystalline PV.

Solar Power Generation

Solar energy can be used to generate electricity in almost any part of the world. The most important thing is availability of some sunlight, which is there everywhere. To generate electricity from sun, you need a solar photovoltaic system. A solar PV system is a long lasting system and can last for 20-30 years. There are 2 types of Solar PV system assemblies that are available:

1. Off-Grid solution: An Off-Grid solution is a system that works independently and is not connected to the Grid. Grid over here means the wiring network from the electricity distribution company that provides you electricity. An off-grid solution generates electricity and stores it in a system of batteries and does not provide it back to the grid. A typical off grid solution looks something like shown below:


In an off-grid solution a PV Module generates electricity that charges the batteries connected to it through a charge controller. The charge controller prevents overcharging of batteries and also makes sure that the batteries do not get discharged at night or cloudy days. There are three types of charge controllers available: 1) Shunt 2) PWM (Pulse Width Modulation) 3) MPPT (Maximum Power Point Tracking). MPPT being the most sophisticated, efficient and thus more expensive and Shunt the simplest, least expensive but less efficient.

The battery bank is the component that needs regular maintenance. Solar systems need deep cycle batteries and cannot be operated on batteries used in automobiles. Life of batteries depends on number of times it is discharged and thus deep cycle batteries are a must. There are multiple types of batteries available in market. The maintenance free ones are the most expensive and they also last no longer than 4-5 years. The other regular lead batteries need to be filled with distilled water regularly for right operation but if maintained well can run for long and are cheaper than maintenance free batteries.

The inverter converts DC current from the battery into AC current that is useful for various appliances in a house. For using with Solar Panels, it can be the same inverter if you have one in your house. There are two types of inverters available in the market: 1) Modified Sine Wave and 2) Pure Sine Wave inverters.
So in case you have an existing inverter solution and you want to have a PV system integrate to it, you can do that by getting a PV module array and a charge controller and connect it to the existing system.


2. Grid Connected Solution: A grid-connected solution is simpler and cheaper to install as compared to off-grid solution. It also requires less maintenance, as there are no batteries involved in it. So essentially a grid connected system is similar to the above diagram without a charge controller and a battery bank. Something like this:


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In this case the electricity generated through the PV array is connected to the inverter as well as the grid. During the daytime when there is ample sun, the electricity generated will be used in the house through the inverter and any extra electricity generated will be sent into the grid. The distribution company can use this extra electricity generated to distribute in other areas. In return the producer of the electricity (the one who has installed the PV system) gets rebate when electricity is used from the grid during the night-time. This system turns out to be cheaper as it does not involve any batteries for storage and thus require less maintenance as well.
As it does not involve storage, such a system is not good for places where there is lot of power outages during night-time.

Sizing of a PV system

Sizing of a solar PV system is not very important if you are installing a grid-connected solution. That is because in case your usage is more than what the system installed can generate, then extra electricity required can come from the grid. If your usage is less than what the system has generated, then the extra will go into the grid and you will get rebates on the electricity you use from the grid.

But sizing can be very important for an off-grid system. The number of PV modules, size of battery array and the inverter will depend on the setup that you want to support on it. As per the standards taken everywhere, it is assumed that on a good day a solar PV system can generate electricity only for 5-7 hours. Which means a 1 kW system will generate 5-7 units of electricity on a real good day.

The best thing that can help you in sizing your system is your electricity bill. If you get monthly electricity bills then just take the electricity units on your bill and divide it by 30 (or 31 based on the month) and you get your daily units usage. If you do not have the same, then you need to find out the wattage of all the appliances that you have and use the formula below to calculate daily units:

Units = (wattage of appliance) x (number of hours of usage in a day)/1000

Sum the units of all appliances and you will get your daily units.

While sizing make sure that you look at the month when you have the maximum electricity consumption. Please note the electricity consumption changes every month (link). Size your system according to the maximum electricity consumption.