Using solar power through an UPS

(This is a small project, but one that should be carried out with responsibility. Do the work only under the supervision of Electrical Engg Department Staff in your college/university Lab).

  

Solar panel prices are coming down but if you include the cost of batteries and solar panels, they are expensive. Products are available in India from a few vendors, but they are not priced like commodity items.  On the other hand, common UPS units are available at commodity prices. They incorporate a battery and inverter and come with power cables, plugs & sockets, all ready for use. The project suggested here is one of connecting a solar panel (SP) providing about twenty or thirty watts of power for a few hours a day to an UPS of the type used with a PC. The schematic below illustrates the idea.

 

A solar panel with a nominal rating of 30 W seems ideal for charging a UPS of the type discussed above. The SP needs only to provide a DC output at the voltage of the UPS battery, which is usually 12 V.  The charge controller has to prevent reverse flow of current and prevent over-charging.  The typical UPS used with PCs has a 600 VA rating and has capacity of about seven Ampere hours (Ah). That provides for a nominal 84 Watt hour (Wh) energy storage. Assuming that we do not want to utilize more than 50% of the nominal capacity before recharging, such a device can deliver 40 Wh per day, enough to power a 5W LED lamp for 6 hours a day and to provide a few Watt hours to charge one or two cell phones. In fact, some UPS vendors offer models with built-in USB sockets that will charge most cell phones. 

The Solar Panel

Those interested in understanding sizing calculations would find the website http://www.batterystuff.com/  quite useful.  The site http://www.wholesalesolar.com/solar-information/charge-controller-article gives some information on the working of a charge controller. ICs for use in building a charge controller are available from vendors such as Texas Instruments.  I would urge students interested in this project to read about the risks of continuing to draw current from a battery when its voltage has fallen below a specified safe level, such as 11.6 V in case of 12 V batteries. Precautions should be taken to avoid under-voltage operation. One way of ensuring this is to build the system around an UPS that provides protection against under-voltage operation.

Social and Economic implications

The LED revolution has made small size solar installations attractive. Five hours lighting per day through a 5 W LED and a cell phone charger working round the clock would be a boon to most rural households. A solar UPS can provide this even without traditional electrification, but benefits from it if available. It makes it easier to live with power-cuts. Lighting based on a solar UPS confers significant advantages to small shops. Remote locations can use solar UPS units to provide some street lighting. Rural hospitals would find reliable lighting very valuable.  This article has had a focus on systems providing power in the range of 5 to 10 W; however, the future will see surging demand for units with higher capacities. At a macro-level, the UPS market in India is already worth several thousand crores of rupees per year; but it largely serves urban customers. Hybrid UPS units that can use traditional electrical supply as well as solar energy are practically absent in rural areas. The technology for putting them together exists and is relatively easy to master. This segment alone is likely to grow into a few thousand crores per year business within a few years. In the early stages, pioneering engineers will put together their own products and provide services. Within a few years, we can expect that small hybrid units will roll off big production lines and be available at commodity prices. The pioneers will see their business continuing to grow despite such mass manufacture, as long as they are present in rural locations to handle retail sales and to provide services.

Apart from providing reliable lighting, this industry will create several hundred thousand jobs all over the country.

Srinivasan Ramani