NOTE:  

A home page with brief technical information, a few images some conceptual detail and relevant links has been placed on the web page: http://home.iprimus.com.au/fredb19/solarhouse/house01.htm

Introduction

This page briefly illustrates the procedure and simple calculations followed to determine the appropriate solar equipment needed to power the Solar House Project on Bribie Island.

• First we specify rough design parameters that will suit our situation.

• Then, we need to get an estimate of the daily power requirements of the household in Watt Hours.

• Then we use this figure to determine a suitably sized renewables design.

BRIEF Overview of Design Criteria

Most designs for completely "stand-alone" solar systems attempt to achieve 5 days autonomy down to an acceptable depth of discharge from the battery systems.  As the subject house is permanently connected to the local Electrical Supply Authority (Energex), we intend to use Energex as emergency backup and to provide emergency battery charging.  This means we do not require as large a battery bank capacity as would normally be desirable for a house with the level of equipment we are installing.  

We consider that approximately 2 to 3 days of fairly heavy household use with zero solar input will be suitable for our installation based on the past history of Energex reliability and our ability and intention to manage household use more strictly for overcast weather.

The passive solar house has been designed with a critical eye on energy efficiency, but the large complement of modern appliances, lighting, smart wiring, monitoring and security systems has been considered an important part of our lifestyle and therefore the house is possibly not an ideal candidate for solar power.   We admit the desire to keep the house as "normal" as possible with all modern electrical conveniences is not easy to design a solar package for, and it is certainly not cost-effective.  However, we want the lifestyle, and I have the interest in the solar concepts - so the money is not really an issue.  Readers will be aware that most solar designs strictly enforce minimal lighting and appliances, and very efficient power consumption practices. This house is not in this category.

In order to use our "state-of-the-art" appliances, Vergola motorised patio roofing (with blade openings that follow the sun), auto garage doors, smart wiring, intelligent control systems, enjoyable sound systems, elegant and relaxing lighting, garden water features and a comprehensive security and fire/smoke alarm system we have opted to accept more frequent charging of the batteries from Energex and we have elected to install more solar panels than the design factors call for.  This strategy should lengthen battery life. We have not included the occasional "off-peak" battery charging costs, but we have given some rough calculations on sell-back of excess power available after the batteries have been fully charged by additional renewables input.

Considerable design effort has been placed in automatic control circuitry and use of the existing off-peak tariff.  The expected advantages to be gained from this strategy have not been included in the design calculations, so additional performance and efficiencies are highly probable.

Our compromising design concept (with a smaller than desirable battery capacity) probably means  more frequent battery bank charging which we can control by selected programming parameters.  However, when solar conditions are right (a regular occurrence on beautiful Bribie Island), batteries will be fully charged very early in the day and export functions will take over and "spin the meter backwards".  This meets our other criteria to minimise our payments to Energex (by selling back excess energy).  To enable sell-back, we need to select a grid interactive Inverter with battery management and charging capability.  We chose an Australian designed and manufactured PSA 5kW unit.

Calculating MAXIMUM Household Daily Power Consumption

Lighting

Total Daily Average Lighting Load =                1,111.5             

Appliances

Total Daily Average Appliance Load =                9,078.5             

Combined Lighting and Appliance Load =                10,190             

Notes:

1.    The table figures are revised occasionally as better estimates are collected.  We would expect the real values to be slightly less than those tabled.

2.    The majority of lighting is low energy Fluorescent.  Exhaust fans & motorised controls taken into account within the lighting loads.

3.    The Liebherr Refrigerator and Freezer and the ASKO dishwasher and front load washing machine are all extremely efficient, low-energy units.

4.    The C-Bus Energy Management, Electrical, Lighting, Control and the Homeminder Home Management and monitoring System run continuously.

Calculating Solar Specifications

Average, calculated household daily power consumption:

 = 10,190 watt-hours

Average peak sun hours (for Bribie Island): 

= 6 hours

Total System Losses = (20% for Battery, 15% for Inverter) 

= 35% of 10,190

= 3,566.5 watt hours Losses

Therefore, total average power consumption of household and system:

= (10,190 + 3,566.5)

= 13,756.5 watt hours

Calculating Required Battery Capacity

For a stand-alone, total solar system, battery capacity should be close to 10 times the average daily load.  Most installations attempt to achieve 5 times daily load.  Due to our circumstances with Energex grid available, we have elected to aim for approximately 3 times average daily load.  Yes! We know this is not ideal, but batteries are very expensive and have a limited life so we have made some compromises....

Therefore, total battery capacity 

= (13,756.5 * 3)

= 41,269.5 watt hours.

We looked at several solar battery systems and selected a 42,000 watt hour battery bank as being a cost-effective unit.  The selected system is comprised of quantity 8 Century Yuasa, 6 Volt, 875 Ampere Hour batteries in series configuration to give us 48 Volts.  These batteries are specifically designed for solar use.  They are a tubular plate, 6 volt wet-cell battery comprised of three, series connected, 2 volt cells encased in a steel container weighing 135 kG each.

For the battery bank to obtain a reasonably useful life, we have selected a maximum allowable depth of discharge of  30%.  This means we will program the Inverter to drop into battery charge mode if the battery bank falls below 70% capacity.

Ie, if no solar input is available, once the battery drops by (42,000 * 0.3) = 12,600 watt hours (which is a little over one full day without any renewables input) the Inverter will (under our selected programming criteria) automatically drop into battery charge mode and bring the battery bank up to full charge status.  We are prepared for extended periods of reduced renewables input and intend to modify household operations if required to assist in extending battery life.

For those interested in battery technology, sizing and selection processes, please see Annex A. at the end of this document 

We do have programming and manual strategies developed to minimise equipment use during prolonged overcast periods and have installed circuitry and permanently connected loads (manual or automatically switched) to take advantage of the Energex off-peak rates.  We have also started to develop energy management functions using C-Bus capabilities which we believe will assist in reducing power consumption.  These strategies have not been considered in our calculations as trials and measurements are yet to take place.

Calculating Peak Output required from renewable components 

Peak output wattage required from renewables:

=    (daily watt hours + total losses watt hours)/(peak sun hours)

= (13,756.5)/6

=2,292.75 watts

As panels will not be on a tracking frame (our frames will be tilted on a quarterly basis ), we will add a nominal 10% to the total peak output wattage required,  therefore

Total Peak Wattage Required from Renewables = (2,292.75 + 229.275)

= 2522.025 watts

Calculating Number of Solar Panels Required

Calculating number of photovoltaic panels required (using Unisolar 64 watt panels)

= (2522.025/ 64)

= 39.4066

Rounding up, 

= 40 panels

Since we have sufficient room to house the panels , and believe we will be running a relatively high household appliance load, we have decided to purchase extra panels to provide some head room.  The decision to purchase an additional 8 panels (total 48 panels) makes a neat wiring connection for the 48 volt system.

Calculating total daily Power Output of Panels

(48 panels * 64 watts) = 3,072 watts output

Therefore daily output:

= 3,072 watts * 6 hours

= 18,432 watt-hours/day

Calculating available power for sell-back to Energex

Total renewables output - peak household wattage requirements (including losses)

(3,072 - 2,522.025) watts

= 549.975 watts

Multiplied by number of peak sun hours

= 549.975 * 6

= 3,299.85 watt-hours/day

= 3.300 kWH/day

At this point in time (to be confirmed soon), Energex buy-back rate is $0.08/kWH  (their selling rate is $0.12/kWH.  They do not "run the household meter backwards, but use an electronic meter with both sell and buy-back odometers).

Therefore, total daily buy-back:

= 3.3 * 0.08

= $0.2640/day

= $24.09 per quarter

Energex has a minimum charge of approximately $24, thus we will obtain a price neutral system.  We can calculate the total expected electrical energy payments to Energex each quarter for the system design:

= (Energex minimum charge - excess energy sold to Energex)

= ($24.00 - $24.09)

= $0.09

WIND TURBINE FACTORS

Until we obtain a substantial log history of the wind turbine energy contribution, we have neglected to use this factor in our project calculations.  The wind turbine, a 400 watt unit, requires a very brisk wind speed to put out 300 to 400 watts.  Up to this point, the wind speed has generally been far too low to provide any useful power output and we have found the unit to be quite noisy (noise is not unlike a cross between a noisy sprinkler and a brushcutter).  Most often we leave the wind turbine shorted which allows it to rotate slowly with no noise.

Our main interest in the wind turbine is its fascination when operating and the visible statement it makes regarding self-sufficiency of power.  We have never expected its contribution to be significant, and as we live in suburbia, we cannot install a larger turbine as it may offend neighbors.

FUTURE DESIGN & MODIFICATION CONSIDERATIONS

The PSA  inverter runs continuously to power a few hidden loads, the C-Bus system, fax, cordless phones and the electronics in the refrigerator and freezer.  The base load without any major equipment operating appears to be in the order of 100 watts.  There is not a great deal that can be done to improve this base load imposition.

Control circuitry, changeover contactors and C-Bus programming has been installed to permit changeover to off-peak power if boosting to the battery system is required.  At this stage the PSA Inverter does not have sufficient user programmable features to permit this function to take place automatically.  PSA is working on this and has promised a ROM change to permit more sophisticated control.

The ASKO washing machine and dishwasher,  both whisper quiet, very energy efficient machines, have programmable control functions that may permit us to explore better use of off-peak power.  Some experiments will be carried out in due course to see if the proposition is viable.  If used, this method will reduce a large chunk of daily watt hours produced by the renewables, and add the consumption to the off-peak import.  The bonus, of course, is that we buy off-peak power at $0.05, then we can sell back our excess at $0.08.  The system has relevant switching to enable switching from off-peak to normal Inverter supplied power or on-peak power at any time. 

Additional solar panels will more than likely be installed if prices for these expensive items drop.  I would expect over the next ten years or so, prices of ever more efficient panels will reduce.

If Energex reliability becomes worse, Energex costs increase or sell-back rates decrease, consideration will be given to the upgrading of the battery bank system to a significantly larger capacity unit.

Annex A.

A short discourse on Batteries.....

Battery Capacity:

Battery capacity is usually measured in terms of Ampere Hours (AH)  It is a measure of how much energy a battery has available for use over time (the same unit can be used to specify how much energy is required by an appliance over time - with these two factors known, we can deduce how long a particular appliance can run on a particular battery).

A simple example: if we have a battery of 100  AH capacity, we could run a pump that draws 5 amps for 20 hours before exhausting the battery (5*20=100).  Likewise we could run a lamp that draws 1 amp for 100 hours.  Obviously, these examples are not recommended, as running a battery flat significantly reduces its life.

What is important is that the amount of energy capable of being extracted from a battery is dependent on the rate the energy is being delivered.  The faster the discharge rate, the lower the amount of energy the battery can deliver.  To give a designer some tools to work with, battery manufacturers use a specification called a "C" rating - typically shown as C5, C10, C20 and C100.  These "C" numbers refer to the time taken in hours to completely discharge a fully charged battery.  For example, a battery's amp hour rating at C10 might be 100AH; at C5 = 90AH and at C100 = 130AH.

Putting this another way, if the battery was slowly discharged over 100 hours it would deliver 130AH of energy, but if you discharged it quickly over a 5 hour period, it would only deliver 90AH of energy.  When selecting batteries for domestic solar installations, the C100 rate is generally used or specified.   

Battery Life:

Contrary to what is generally spoken, a battery's life is not measured in years, it is measured in "cycles" ie, the number of times a battery is cycled between a discharge level and then recharged back to its original level.

Cycle depth or "Depth of Discharge" (DOD) refers to the level to which a battery has been discharged and is expressed as a percentage of total battery capacity.  For example, a 100AH battery which has had 10AH drawn from it before it is recharged has experienced a DOD of 10%.  This is a very important factor in determining the life of a lead-acid battery.

The number of times a battery can be be cycled before it dies is directly related to the DOD.  Thus it follows that a battery bank should be sized so that any cycling is shallow, say a DOD of less than 20%, with the occasional worst-case DOD of 50%.  The worst case is generally when a battery is "overworked" or in the dead of winter during inclement weather.  To assist in sizing, you should refer to the Manufacturer's specifications - generally, quality batteries generally have in their specifications a table or graph of life vs a range of DOD's.

Battery Types:

The batteries normally selected for domestic solar systems are lead acid types, either "flooded cell" or "wet batteries", and "sealed cells", often called "valve regulated lead acid cells" (VRLA) or "sealed lead acid batteries" (SLAB).  Gel cells are a particular type of VRLA which has its electrolyte suspended and immobilized in a  thixotropic gel.  Another type of sealed cell is the "absorbed glass mat" (AGM) type which has a porous mat separating the cells.

Battery Efficiency

Due to gassing and other chemical reactions lead acid batteries are never greater than about 90% to 95% efficient.

Battery Problems:

Lead acid batteries suffer from a condition known as "sulphation".  When partially discharged for an extended period, lead sulphate, which is normally formed during the chemical reaction taking place during discharge, crystallizes into an insoluble form which results in an irreversible loss of capacity.  Sulphate crystals show up as a white or brown deposit on the plates and in the bottom of the cell containers (clear plastic battery cases are good for displaying what goes on inside a cell!).

Stratification can also be a problem,  This often occurs when a battery is not being cycled, or only lightly cycled and the electrolyte stratifies into layers of differing densities... This can reduce the life of a battery by accelerating plate corrosion.  The remedy here is insulate the battery from a cold concrete floor, and regularly "overcharge", causing gassing and stirring up the electrolyte.

Typical Battery Specification:

I have used the battery type that has been used for our project as the example here:

Century Yuasa Solar Sun SSR 675-6, 6 volt lead acid

AH rating at C10 = 662,  at C100 = 875

Cycle Life @ % discharge  (years in brackets. assuming one cycle per day)

10% DOD = 2500 cycles (6.8 yrs)

25% DOD = 1800 cycles (5 yrs)

50% DOD = 1200 cycles (3.3 yrs)

80% DOD = 900 cycles (2.5 yrs)

Please feel free to contact us if you wish to comment on the project.   Click on the mail box below if you wish to send us an e-mail message:

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