rowanlangley (rowanlangley) wrote in green_power_gen,

Solar re-design

Having been both working full time in the day job and for the last 18 months full on also in a "Pro Bono" job involving the renovation of the former Spa Road Library in bermondsey, the solar power research project has taken a back seat.

With the prospect of some new research funds in the form of a long service award from the day job - yes 25 years in the firm, things are due for a bit of a good stirring up and re-building.

The system which basically settled into shape in 2007 gave some good years being off grid for around 8 to 9 months each year. The main weaknesses were having a series - parallel battery bank, chosen to allow scaling up as budget allowed, combined with a charge controller which is just a fairly clever shunt stabiliser. The consequences of these decisions have been these :

Any battery which deteriorates from age and fails to hold charge, or develops and leak or short accross a cell then starts draining energy from the good sections in the battery bank.

Current sharing under charge or discharge has been at the mercy of the general health of the worst battery in each of the three pairs.

The solar output voltage has been hauled down to the battery bank voltage, a mis-match which has left around twenty percent of the available power unrecoverable. A consequence of this has been that the batteries have not always been given the best charging conditions and have aged rather more quickly than I would accept for a system which was to have any possibility of being a commercial proposition.

The experience with the system which was installed in 2009 in southern Scotland (derived from the system here in London) has been encouraging. This system replaced the shunt stabiliser for one which both tracks the maximum power point of the solar array and performs a voltage step down / matching process between the array voltage and the battery voltage. This has three consequences.

Firstly all the potential power from the aracy is recovered. there is a small loss in the conversion electronics, a few per cent, which is outweighed by not having the 20 percent loss from the electrical mismatch.

The second is that it has allowed the array to operate with higher voltage and low to medium current, which reduces losses in the DC cabling from the array to the electrical equipment.

Third, for the design process, there is greater freedom in selecting the panels and aray connection scheme as there is no longer the requirement to choose panel maximum power point voltages close to but above the battery bank voltage and instead choose panels on area, and price per watt, and in particular various types designed for large grid connected systems which give a significant saving.

The first big change and improvement is incorporating the facility to work either with a charge controller and battery bank or a grid connect inverter. While I am away on holiday it won't take a great deal to keep the battery bank fully charged, and rather than not use the remaining sunlight during the day, it makes more sense to be able to dump it to the grid for a bit of beer money.

The major changes are at the input side. The new design the same type of charge controller as the system built in Scotland. The dual inverter configuration is being retained.

The input is equipped to take an array in up to three sections. In keeping with the guidance in BS7671:2008 both poles of the input from the array are protected with over current devices. Following the circuit breakers comes a bank of three relays. The array section outputs go to the commons, the normally closed outputs go on to the charge controller via a 40A circuit breaker, while the normally open outputs are wired so that the array sections connect in series (the preferred configuration for a grid connect inverter) and then a double pole isolator ready to go off to a G83 inverter if the budget stretches that far.

The next big change is the battery choice. With the budget to go straight for the full size system a series battery bank is the better option. With the occasional loads of 2.4KW being supported (Washday) the battery bank size of 330AH on the old system was quite inadequate. The new design is aiming for at least 400 AH and going for a set of 4 6v Rolls 4000 series batteries with a capacity at C20 of 450 AH.

A typical base load of the house is around 150 watts, which at a nominal 24 volts corresponds to a discharge current of 6.25 amps. The same batteries quote a capacity at C100 of 600AH. (C100 is a discharge current of 1/100 times the capacity) This is very convenient indeed as the anticipated base load current from the batteries is around that C100 figure of 6 ampere. This promises to be much kinder to the batteries than the previous system where the discharge current was rather higher than the C100 figure by a factor of 3.

Having spent a fair bit on panels over the last five years, I would quite like to keep my existing array and not have to start again from scratch. This appears possible, with the following considerations. The array has to be able to connect in parallel for battery charging purposes, which require each section have the same nominal voltage output. The sections also have to connect in series for the grid connection function, for which each section must have the same short circuit and maximum power point current.

The array presently mixes polycrystalline with electrical factors of 7A maximum power point current and 42 volt open circuit voltage with amorphous silicon units with comparable open circuit voltage but 3.5 A maximum power point current. The number are 6 polycrystalline panels and 4 amorphous. The first stage is an array configuration using what I have now. By connecting the amorphous units in two parallel strings I have something close enough to the characteristics of the polycrystalline panels to make an array of two sections, each with an open circuit voltage of 84 volts and a maximum power point current of 7 amps, satisfying the conditions for both series and parallel connections and the requirement of a maximum power point tracking controller which is that the array voltage needs to be significantly above the nominal battery voltage

The re-design and re-build began last night with the wiring of a new equipment cubicle to house circuit breakers and related items. This gives a complete system ready to connect panels and charge controllers as the orders start to be delivered over the next few weeks. At the same time Q-cad has been busy laying out the footprints of the proposed batteries over the available space in unused cupboards, existing equipment housings and led to the choice of equipment in two housings, 1 housing 600 (w) x 400 (d) by 1000 (h) for the batteries with internal shelving in ply, and a second of the same size to hold the mcb / relay cubicle, inverters, charge controllers and output connectors. The cabinets will probably move to my kitchen to take advantage of a concrete floor and having the garden the other side of the wall to allow for ease of wiring.

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