The Solar Bucket

The solar bucket (an alternative description being a plastic box full of wires) was designed to conduct a site survey before making a decision to install solar panels for electrical power generation and/or domestic water heating.  The process started around November 2007 and will probably continue until March/April 2009.  The total cost will have been around £150.  This has to put in the context that full scale installations start at £3,000 for water heating and £7,000 for electricity.  Before spending significant sums of money I want to be certain that the investment makes ecological and economic sense.

The design is evolving, it started life in November 2007 as simply a 12V computer case fan connected to a 1.5W solar panel.  During December and January the panel generated enough energy for the fan to turn for between zero and three hours/day.  This modest result satisfied my first objective that would do something useful during the winter months, exactly how useful will not be clear until the winter of 2008/9.  In its current form, the bucket uses a small sealed lead acid battery to "measure" the energy captured by the solar panel.  During the day, the battery is charged by the panel, in the evening the voltage across the battery is measured, overnight, the battery is discharged gently using one or two 100Ω 10W resistors.  First thing in the morning, the voltage across the battery is measured.  The difference between the evening and morning voltage readings indicates the energy captured by the panel. 

The circuit diagram is shown below.  The day to day results are shown on a graph on this site's home page.

Design Objectives

Apart from simplicity and low cost, the design objectives for the bucket were:

Minimal Observations

A complex observing routine is unlikely to be maintained over a prolonged period, the current regime simply involves two quick and simple meter readings, one in the morning and one at night, at each reading, the switches can be toggled to charge/discharge as appropriate.

The Instrumentation did not alter the performance of the system.

It is attempting to add sensors and link these to a computer, not only does this add complexity, with small energy flows, the energy absorbed by the instrumentation may distort the results.

Separate the Charging and Discharging Phases

There should be no interaction between the charging  process and any other load applied to the solar panel.

At the time of writing, no attempt has been made to optimise the sizing of the components.

Calibration

The battery was calibrated by charging it to its maximum capacity, then discharging it in stages, after each stage the voltage was allowed to stabilise.  An estimate of the current drained during each stage was used to create an idealised relationship between the stable voltage across the terminals and the state of charge.  This is shown in the graph below:

It is hoped that by cycling the cycling the battery between 12.1 and 12.8V the degradation of the battery over a nine month period will be minimal.

Evolution

The design is evolving to take account of lessons learnt.

Design an build controller

Based on experience with the existing battery and controller, the charge controller should ideally disconnect at 14.4V and reconnect at 12.9V (below the fully charged stabilised voltage.  During discharge, the battery should disconnect at 11.5V or higher and reconnect at 12.0V or higher (above the fully discharged stabilised voltage.

Replacement of Fan with Inverter

The resistors provide a gentle load (max 250mA declining to around 80mA) to discharge the battery, however it would be more interesting to use the energy to power something like a desk lamp with a 5W CFL powered from a 12V DC to 240V AC inverter.

Page Updated: 11-Jul-08

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