Saturday, October 27, 2012

Solar power for homes

Power shortage and blackouts in various parts on India in 2012 made me think about solar power generation at home. I had various questions in my mind: How to produce solar power?, What is the cost and return on investment? etc.
I will try to answer the first question in this article.

Electricity can be generated from sunlight using photovoltaic cells[1]. The amount of power generated depends on various factors like solar radiation received, efficiency of the solar cells, temperature etc. 
Photovoltaic cells produce DC power. Most of the home appliances run on AC power. So a DC to AC inverter is required to use the solar power. 
As solar power is not available during night, batteries are required if backup is desired.

Various options for using solar power at home
Solar power system can be of different types: off-grid(not connected to external electricity supply) or on-grid, with or without battery backup. I've considered the following:
  1. Off-grid + battery backup
  2. On-grid - battery backup
  3. On/Off grid + battery backup
Off-grid + battery backup
The power generated by solar panel is used charge the batteries. A charge controller is required to cut-off the power when the batteries are fully charged. The output of the batteries is fed into a inverter which converts DC(e.g: 12V,48V etc) to 230V, 50HZ AC. This can be fed to the home appliances.

On-grid without battery backup
This system makes use of an on-grid inverter. This works in 3 different ways:
  1. If power from panel == load, no power is drawn from the grid.
  2. If power from panel > load, remaining power is supplied to the grid.
  3. If power form panel < load, balance power is drawn from the grid.
Power can be supplied to the grid only if the local power company permits you to do so. This is called net-metering and is available in some countries(now aware of such policy in India).

On/Off Grid with battery backup
As the name suggests, this is a mix of the first two options. The difference from option 2 is that this charges the batteries first and supplies to the grid only after the batteries are fully charged. The power from batteries can be used when both solar power and grid power are unavailable.

Comparison of various options
I've compared the various options on the following parameters:
  1. Investment cost
  2. Utilization (whether 100% solar power received can be used)
  3. Maintenance cost
  4. Limitations

Installation Cost Utilization Maintenance Cost Limitations
Off-grid Very High (due to batteries) < 100% High (due to batteries) Requires very high capacity
to replace grid (4KW for every 1KW load)
On-grid Medium 100% Low Net-metering is not available in India
On/Off-grid Medium to high (depends on batteries) Close to 100% High (due to batteries) Net-metering is not available in India.

An off-grid system is too expensive and seems to impractical for home use. On-grid system is the least expensive and utilizes all the power generated but requires a government policy to buy the power back. If backup is desired an On/Off-grid system with limited backup (to keep the cost under control) would be recommended.