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 Maximum 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.
Yes that is the bottom line isn’t it ? Should be straight forward to answer if – the Ifs and but(s) appear and the issue becomes too confusing. Let me explain why.
Installed Wp Vs What we actually get on field.
Wp rating of the Solar Panel is what you pay for. However the Wp rating is based on Standard Test Conditions (STC) – which differ widely from the conditions in your installation. For example STC assumes 25 deg C for the Solar Cell temperature. What you have on your roof top is an ambient temperature of say 35 deg C and the Solar cell temperature operating at closer to 70 deg C ! As the output of the Solar module drops with increased Cell temperature -the maximum that you can get on the roof top is already lesser than what the name plate suggests. A good rule of thumb is about 75% of the rated Wp that you can get as the maximum.
Now that you have calculated the maximum Wp that can be extracted from the Solar Panel- you need to know how many units of electricity the panel would extract per day.
In India – you can assume 5.5 hrs equivalent of Wp – roughly for the calculation of Units generated. The extract-able energy from the Solar PV panels varies through the day and there would be a time close to noon when this is close to the maximum that can be extracted from the Solar Panels. Let us say Sun is available for 10 Hrs. When we say 5.5 hrs of Wp equivalent – what that means is that the amount of energy that can be extracted in Ten hours is equivalent to having 5.5 Hours of maximum Wp extracted.
Based on the thumb rule above and the 5.5 hrs Wp equivalent – an one kWp module will deliver approximately – 5.5 X 1 X 0.75 = 4.1 kWhr Or about 4 units per day .
The energy generated is only one side of the equation. It is useful only if the energy generated can be consumed !
Ex-tractable energy Vs What is actually consumed
In the absence of Grid tie option – where a solar installer can export excess energy into the local grid and can get paid for the same- offgrid installations can end up wasting extractable energy unless Solar Panels are undersized. There would be periods when the batteries are full and loads not enough to consume all the available energy from the installation. In this case – the charger ends up extracting lesser than the maximum power available – thus reducing the returns from the installation. Its difficult to match the load timing to the Solar power availability. Note however – this author feels that if the objective is to obtain sufficient back up – then that needs to be addressed first. That need would drive the panel sizing and the ‘wasted’ energy is not such a bad thing.
Yes – Installing Solar PV does save money.
Solar energy when used – saves on the power otherwise drawn from the grid for charging the batteries or powering the loads. However most offgrid systems still have a payback period of > 8 Years and compares well with alternatives like Diesel and not having power at all. In states like Kerala where the rate per Unit of electricity for increased consumption is much higher – Solar PV would deliver a shorter payback.
* Picture of the electricity Meter is from BESCOM site.
This is another very commonly asked question.Here are some guidelines to determine the size of install in India.
Money as a constraint : If Rooftop schemes with reverse or net metering is available in your locality ( not unless you are in Gandhinagar for example http://mnre.gov.in/file-manager/UserFiles/presentations-pwc-workshop-06092012/GEDA.pdf) install as much as you have cash to spare (at today’s prices its about Rs 120 Per Wp , landed cost inclusive of the electronics , structures etc) – till you hit the next constraint which is space.
Shade free space as a constraint : You will need about 100 SqFt per kW installed with area free of shade , south facing.
However -rooftop net metering is going to take some time to become a reality. If you still want to add Solar PV (while you keep following up with the local utility about reverse metering ) - its usually because a) You think you want to do some good to the environment b) you are in the unfortunate town or village where power cuts are the norm.
A). For someone who is looking to do ‘good’ – and cannot export the power generated to the Grid – you look at the two constraints : Money and Space to determine what is the size of install. You may have to rewire the connections – and add storage because of the nature of Solar PV power production. Accept the fact that not all extractable energy can be used all the time.
B) Solar PV as a back up power supplement : Unless you are prepared to ’ waste’ otherwise extractable Solar energy – the size of the install is much lesser than the total connected loads. Sizing the Solar PV is related to the hours of power cut, the size of the batteries and the average power consumption in your home. Sadly there are no electronics that manage the usage of Solar PV with Grid, loads and back up in a way that is seamless to the end user- yet. Till such solutions are available – be clear on whether you want assured back up ( even in the face of ’ wasting extractable solar energy) Or Maximal usage of Solar.
We will look at some simple math in a later post for the sizing.
Recently Bridge to India Folks put up a Solar Calculator – Check this out : http://indiasolarhomes.com/calculator-2/