Adding Solar PV to Existing Inverters

How to get real savings in Electricity bills and keep the batteries safe!

Electricity supply in India has improved enormously in the recent past. The Southern region is expected to have an high Peak shortage (Power cuts !). The Eastern region is expected to have an energy surplus in 2015-161. With increasing power availability, consumers are looking to add Solar PV to help in reducing electricity bills. In the absence of grid tied options, adding Solar to existing Inverter based back up makes economic sense. Adding Solar to existing Inverters allows the reuse of the investment already made in Inverters & Batteries.

With this approach, the Solar PV panels & a Charge Controller that controls the charging from both the existing Inverter and Solar PV is retrofitted to the existing Inverter. The combined system is expected to work as a single unit, maximizing the use of Solar PV and providing sufficient power back up in case of power cuts.

Most retrofit charge controllers control the Mains input to the Inverter, switching off the Mains supply, when Solar PV is available so that the loads connected to the Inverter are powered from Solar PV (and the batteries).

A good ‘retrofit’ charge controller MUST control the Charge Rate to the batteries if they aim to really save electricity bills with the addition of Solar PV. To get real savings in electricity bills, the Solar PV addition must generate enough energy to completely service the day time loads connected to the Inverter. For example, in a typical home, the day time energy consumption might be 2-3 Units (kWHr). A kWp of Solar PV panel would generate about 4-4.5 Units per day. A Solar array suitably sized would be capable of generating the required energy. The conflict arises becase of the size of battery back up available. Most existing Inverters would have Lead acid batteries of ~ 150Ah capacity. Lead acid batteries must not be charged at a rate higher than 10 -15% of their battery Ah. When charged at an higher rate, the batteries reach gassing Voltage before they are fully charged.However, a Solar PV array to take care to day time loads might deliver Current that is higher than the optimal Charge rate. Installing larger batteries is expensive & not very useful. Installing smaller PV capacity is counter productive – in the end, the energy deficit would be bridged by the use of Grid supply and will result in increased bills. Not what the user wants either. The retrofit charge controller Must monitor the net current delvered to the batteries and Must control this current to the optimal charge rate – so that Loads get the required power and the batteries are used safely as an effective buffer for the variations in load and generation.

Therefore a good solution to add Solar to existing Inverters to be of real benefit to users must allow sufficient Solar capacity to take care of day time loads and keep the batteries safe at the same time.

1Page i.. Anticipated All India Power Supply Position for the Year 2015-16. Load Generation Balance Report 2015-16. Central Electricity Authority.

 

System Integrator Question Series – What Panel should I use with my PWM charger?

The other day, in a meeting with a system Integrator from Trivandrum (He has been a great supporter of our products and has given us tons of valuable feedback and some good business too) we were asked if it was OK to use a 60 Cell panel with 24 V battery system and a PWM charger.  We had to tell him that it would be better to use a 72 Cell Panel with a 24V battery system esp with a PWM charger. (Similarly a 36 Cell panel with a 12V battery system).

The reason is simple.The Current – Voltage curves of the 60 Cell Panel seem to indicate that the MaxiEldora250Pmum power point is at 30V. Given that the 24V batteries are going to operate at a maximum of 28V and therefore the Maximum power Voltage is higher than the highest operating battery Voltage, (See the I-V curve on the left – this one is from the Panel data sheet of Vikram Solar http://www.vikramsolar.com/eldora-prime-series.aspx)  the PWM charger ought to have no difficulty charging the batteries at all the various Battery voltages of a 24V battery system. However – these curves are at STC (Standard Test conditions ) and even at STC,  at lower Solar availability, the maximum power point has shifted to a lower Voltage value. And real life conditions are very different from STC . For one, the ambient temperatures when we get 1000W/m2 of Solar power is likely to be  much higher than 25 deg C that the STC uses. The actual cell temperature is probably close to 65- 70 deg C. Panel data sheets refer to the drop in Voc and the Vmpp with a temperature coefficient. (e.g the same Panel data sheet the Temperature coefficient is mentioned as -0.31% per deg C. Apply that to the STC values and we find that the actual maximum power point is now nearer 24-26V and not the 30V that we see in the standard I-V curves. Now with battery voltage at 24V or more – the PWM charger is forced to operate to the right of the knee instead of the left of the knee.  ( in the curve above , the maximum power point is close to the knee of the curves).

The problem with operating at the right of the knee instead of the left of the knee is obvious once we look at the curves above. On the right of the knee, we see that the curve is dropping sharply and that means that a small shift to the right on Voltage will mean a large drop in available Current – and therefore the Power ( Power: multiply I and V). Whereas if we operate to the left of the knee- the value of Current (I) does not change much and we end up getting more power in the exact same conditions. We may even be operating on the knee and getting the maximum power with the PWM charger if the knee is ‘right’ enough. Take a look at a similar IV curve for a 72 Cell panel below. Even under real life conditions, we are likely to operate at the left of the knee and that is why it is advisable to use a 72 cell panel with 24V battery systems and 36 cell panels with 12V battery systems. Eldora200P