PWM, MPPT and measuring real output

Yesterday I received a call from a system integrator – who has been installing solar systems using PWM charge controllers and is looking at switching to MPPT charger.  He has an experimental set up with a 200 AH batteries, 48V, 2500 Wp Solar panels : 250 Wp X 10 nos , a PWM charger and a MPPT charger which he had acquired from his recent visit to Bangalore. (Its not from AMBRT though)

He was aghast that contrary to his expectations the back up power from PWM charger was longer than that of the MPPT charger.

Questioning him further – revealed two common issues with comparison. 1) Unlike the PWM charger , the MPPT charger is expected to have a different input Voltage and current compared to its output. If the MPPT charger is working to its full potential- then the Voltage in the input is likely to be the Maximum power point Voltage – Vmpp.  If you would like to compare two chargers – they are ideally done in parallel  connecting to the same battery voltage and are extracting power from two arrays under same conditions of solar incidence.  If this is not possible, then calculate the total energy delivered – on a few days – hopefully with similar solar incidence.

Note : The wiring of the array using PWM charger may not work for the MPPT charger. In the above example, with just two panels in series, the Voltage of the array could be in the lower end of the input Voltage range of the MPPT charger , BUT the current  might be higher than the input current limit. Therefore the array may have to be rewired with 3 Panels in series instead of two in series.

2) Battery Voltage matters. As I found out in this case, the MPPT charger was not pushing the battery voltage to beyond 54V – based on settings in the charger. The PWM charger on the other hand was pushing the battery voltage to much higher – so definitely not an apples to apples comparison. What was probably happening was that the MPPT charger was backing out when the battery voltage reached 54V. The MPPT charger was then regulating to keep the battery voltage nearly constant , allowing the battery to draw as much current it wanted. The MPPT charger was no more operating in the MPPT point.

so what is the best way to compare both the chargers?

If the array cannot be made parallel and each half connected to a different charger – the next best thing is to

a) Connect the charger to the batteries on two different days – note the sunlight conditions and judge if they are similar.

b) Log the Battery Voltage and measure the Current at the output of each of the chargers in regular intervals. – Say every ten minutes – through the day.

c) Make sure a load is connected to the battery so that the battery is not fully charged. For example in the 48V system , adjust the loads so that the Voltage of the batteries are maintained at around 51V.

d) Multiply V and I and sum the values to determine the amount of units ( kWHr) of energy delivered in the same time frame. This should give you an approximate estimate of the performance of the two chargers.

Note : For the MPPT charger – the wiring of the panels may have to be changed due to the different input current ratings.

How much Solar PV should I install in my home

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/

Does MPPT make sense in Indian Conditions

As we interact with a variety of different solar system installers, we have got this question repeatedly – does MPPT make sense in Indian conditions.

The implied question is – MPPT controllers cost more than the PWM counterparts – is it worth the while to add this cost to the installation.

First : The difference between MPPT and PWM chargers. There are ton’s of literature on this subject on the net- briefly  PWM chargers essentially short the panels to the battery and the power from the panels is extracted at the battery voltage. i.e Power extracted is Vbatt X I – which is the current dictated by the PV panel’s characteristic I-V curve at Vbatt. This Voltage and therefore Current I, when different from its Maximum power point Voltage, the PWM charge controller ends up extracting lower energy from the Panel compared to the maximum possible. The panels are matched to the battery voltage and the Vmpp of the panels chosen is always higher than the highest battery voltage. That is why, for example for 12 Volt battery systems, the so called ‘12V panels’ have Vmpp of 17V.

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In Indian conditions, the cells in the PV panels are operating under ambient temperature conditions that are much higher than the ideal name plate Wp conditions i.e 25 deg C. The maximum power point voltage shifts to a lower value – and hence closer to the battery voltage.  The difference between the Vmpp and Vbatt is considerably reduced . Hence the extra power extracted using a MPPT charger is reduced. 

Note that the MPPT charger is going to deliver extra energy – BUT the addition over PWM chargers is reduced because of typical India conditions (at say ambient of 36 deg C and NOCT – 47 deg – the difference is about 15% ) For a 100 Wp system – this is only about 10 Wp in typical conditions and based on today’s Solar PV prices, that allows only ~Rs500 extra for a MPPT charger. However as the Wattage increases – the head room for MPPT is better – for a 500 Wp install the extra cost afforded for a MPPT charger is ~Rs 2700/- and so on. That is to get the same performance of a MPPT charger with a PWM charger – the installer has to spend Rs 2700/- worth of PV panel extra in a 500 Wp installation.

However this calculation completely ignores the other benefits of using MPPT chargers.  With MPPT chargers, the need to match the PV panel Voltage with the Battery Voltage is no more present. The system integrator can choose mass produced PV panels that cost lesser on a per Wp basis.

Further if the MPPT charger is designed well – the input range of acceptable voltages is high and this means that the input can be operating at higher voltage – thus reducing the wiring and connector costs.

In conclusion : MPPT chargers can have the effect of reducing the system costs and deliver more energy even in Indian conditions.

How a good MPPT charge controller (e.g. REhub) reduces system costs

REhub  is based on an high efficiency MPPT charge controller with the ability to accept a wide input MPPT range ( e.g 17 – 65 V for a 12V battery system).

System_integrators can use this to their advantage by

a) Selecting PV panels that are mass produced and therefore of lower cost. E.g to set up a 400Wp installation – using 4 X 100 Wp panels cost more than using 2 X 200 Wp panels. Yes, the 100 Wp panels are selected in installations today because of the need to match the PV panel output voltage to that of the battery system voltage – not needed any more with REhub.

b) Wire the panels in an all series arrangement. Typical Voltage at the Maximum Power point ( Vmpp ) for a 200 Wp , 60 Cell panel is about 30V. Wired in series the combined voltage is at 60Vmpp. REhub converts the input power at Vmpp to Vbatt ( the battery voltage) efficiently. The higher input voltage would mean that wiring costs are lesser compared to having all panels in parallel and operating at 17 V.

c) Lesser civil work and structurals on the roof top. Lesser panels : lesser cost .

Check out www.amberroot.com/rehub for more details.