4. Design
4.1 The design process
The system design process consists of four major steps. These are:
- Initial estimates
- Site survey
- System sizing
- Component selection
The order in which these are performed will depend on the amount of information available in advance and factors such as the ease of visiting the site. If detailed information regarding the location of the site and the intended loads are available then it may be possible to size the system before a site visit takes place.
4.2 Initial estimates
Before the commencement of the design process proper, you will need to have at least a rough idea of what you hope to achieve, for example: “To provide lighting and refrigeration for a holiday home”. From this it will be possible to produce initial estimates to feed into the system design process. The following paragraphs expand on this example.
4.2.1 Load estimates
In order to estimate the load requirement we need to get an idea of the type of usage the system will be put to. For the above example of a holiday home we should be able to discover how many rooms it has and how many people will be likely to be in residence. If we make the following assumptions:
- There are 3 rooms, one of which is a bedroom, therefore;
- There will be no more than 2 people in residence.
Then we can estimate the lighting and refrigeration as follows.
4.2.1.1 Lighting
From the above we know that there are three rooms, so the total number of lights required is 3. Now we need to estimate the average daily usage of each light.
The first thing we can deduce is that, if there are two people then there need not be more than 2 lights on at any one time. Then we can make an estimate of the amount of time between darkness falling and the residents retiring. Let us say that is 8 hours.
Now, let us assume that the occupants spend half of this time together. In that case one of the lamps will be on for half of the time (4 hours) and the other for all of the time (8 hours). So this gives us a figure of 3 lamps and a total of 12 hours, hence each lamp is on for an average of 4 hours per day.
Lastly, you need to estimate the power consumption of each lamp. This is a matter of picking a type of lamp which you think will be suitable by examining the lamps available to you. For this example let’s assume that an 11 Watt, 12 Volt fluorescent lamp is selected.
4.2.1.2 Refrigeration
Estimating the refrigeration requirement is rather more straightforward. All that is necessary is to find a suitable (12 Volt) refrigerator in a manufacturer’s catalogue and look up its daily energy consumption. This will be determined by the ambient temperature so an estimate of that will be helpful. For the purposes of this example let’s assume an energy consumption of 600 Wh/day at a 25°C average.
4.2.1.3 Other loads
The energy requirement for any other loads is calculated in the same way as that for the lighting. The power consumption of each item is multiplied by the number of hours it will be used in a day to give the energy consumption in Wh/day.
4.2.1.4 Phantom loads
Phantom loads is the name given to those appliances which use power even when they are switched off. Example include audio-visual equipment such as televisions and video recorders and anything which has the power supply built into the plug. Anything that falls into this category should be unplugged when not in use or provision made to switch off its supply. However there may be appliances which need to remain plugged in. The standby consumption of any such appliance must be treated as an additional load which is in use for all the hours of the day that the appliance itself is not in use.
4.2.2 Location
The intended location of the system will determine the solar resource which is available. This in turn will allow the size of the solar array to be calculated. For the purpose of this example, let’s assume that the holiday home is in northern Portugal.
4.2.3 First iteration
From the initial problem:
To provide lighting and refrigeration for a holiday home
we have now arrived at:
To design a solar power system to be installed in northern Portugal, to power three 11 Watt lamps for an average of 4 hours per day and a refrigerator with an energy requirement of 600 Watt-hours per day
Following the system sizing process (section 4.4) will show whether this is a practical system. If not, then make changes to the requirements and start again. For instance, in this example the refrigerator consumes far more energy than the lighting. If the system is likely to be too expensive, then consider using a gas refrigerator instead. The capital cost will be lower, but there will be a fuel cost to take into account.
4.3 Site Survey
In some cases it may be necessary to complete the design without having visited the site, in which case certain assumptions will need to be made. In these circumstances it would be advantageous to obtain photographs of the site and the surrounding area if possible, or failing that a detailed description.
The various points of the site survey are covered in the following paragraphs. It will be helpful to take photographs of the site for reference later. Pay particular attention to those areas chosen for the various system components; as the design progresses these will be invaluable.
4.3.1 Shading
The first and most obvious check is to ensure that the sun actually shines on the site. From the projected position of the system survey the horizon over the entire arc of the sun. In the northern hemisphere you should be looking towards the south, east and west and in the southern hemisphere the north, east and west. Very close to the equator the sun passes virtually overhead, so only the east and west are important.
You should be looking for anything which will shade the solar array at any time of the year, including such things as:
- Trees. If it is winter when you visit, remember that some trees will look very different in summer. Also include sufficient space for 20 years of growth.
- Hedges. Again allow for these to grow significantly during the life of the system.
- Mountains and hills. Remember that the sun will be much closer to the horizon in the winter. If it is summer when you visit, ask someone local where the sun rises and sets in the winter.
- Buildings. Ask around to ensure that no building work is planned which will obscure the site.
- Climate. Find out if there is anything unusual about the climate in the local area such as sea mist.
Try to imagine what the site will look like all the year round and in years to come. You may find it helpful to make a sketch of the surrounding area for later reference.
4.3.2 Array location
It will be necessary to find a position for mounting the solar panels or solar array. If the system is to be installed in a building, then it is common for the solar array to mounted on the roof of the building as described below. If this is not possible then an alternative site will need to be found.
4.3.2.1 Roof
The ideal is for a roof with a slope towards the south if in the northern hemisphere or the north if in the southern hemisphere. The angle of this slope to the horizontal needs to be about equivalent to the angle of latitude plus 15°. It is very unlikely that these conditions will be met, however the roof is still likely to be the best place if it slopes in roughly the right direction or is flat. If it is flat, however, it will be necessary to arrange some type of angled support such as that used for ground mounting.
If possible gain access to the roof in order to survey it more thoroughly.
Check the following:
- Shading. See section 4.3.1.
- Direction. Use a compass to check what direction the roof slopes towards.
- Angle of slope. Use a spirit level to measure the angle of the roof from the horizontal.
- Material of construction. If necessary also check underneath the roof to see what fixings will be needed and ensure that the structure is strong enough to support the weight of the solar array.
- Area. Measure and record the dimensions of the usable part of the surface of the roof. Estimate whether this will be sufficient for the size of solar array that is likely to be needed.
If it appears that the roof will not be suitable then it will be necessary to find a site for an alternative form of support.
4.3.2.2 Ground mounting
In the absence of a roof or similar structure to mount the solar array on it will be necessary to use some form of support structure.
Solar equipment suppliers sell different types of structure or it may be possible to fabricate a support on site. There are two basic types as illustrated in figure 14:
- Ground mounted, where the structure is a frame mounted on the ground which requires a foundation, and
- Pole mounted, which can be attached to an existing pole or a pole erected for the purpose.
The survey should take account of:
- Shading.
- Ground conditions, for the purpose of building foundations.
- Available area of ground.
- Distance from location of batteries for cable sizing.
- Any suitable poles.
4.3.2.3 Other options
There may be other mounting systems available. For example, figure 16 shows a system where the solar panels are mounted with other system components on a south-facing gable end. If there is no potential for roof or ground mounting then it may be that there is another solution which will suit the needs of the planned installation.
4.3.3 Batteries
4.3.3.1 Location
A suitable position must be found for the batteries. This may be a room within a building, a separate building or a place where some kind of housing can be erected. The following conditions need to be met:
- Environmental protection. The batteries need to protected from rainfall, direct sunlight and extremes of temperature.
- Ventilation. All lead-acid batteries, even sealed types, need to be adequately ventilated in case of gassing.
- Protection from sources of ignition. When under charge vented leadacid batteries give off an explosive mixture of hydrogen and oxygen, so must not be exposed to any sources of ignition such as naked flames.
- Distance from location of batteries for cable sizing.
- Personal safety. Because of the explosive gasses given off and the potential for extremely high currents, batteries must be kept in a secure place away from children.
Consideration should also be given to the likely location of the other system components. The aim should be to ensure that the cable runs are kept as short as possible.
4.3.3.2 Mounting
Where a suitable location indoors has been identified, it will often be acceptable to place the batteries directly on the floor. If not, it may be necessary to mount the batteries in a battery box or on racking in order to make best use of the available space or to offer them adequate protection.
Consideration should be given to the likely shape of any such boxes or racking, and the area measured to ensure that the batteries will fit as intended.
4.3.4 Control equipment
It is usual for the controller, inverter and other control equipment to be wall-mounted. An indoor location will be needed, as close to the batteries as possible. There is often a restriction on how long the battery cables can be, so it is important to ensure that this can be met.
Inverters in particular are often quite heavy. Assess the chosen wall so as to ensure that it will be able to take the likely weight of the equipment.
4.3.5 Loads
The site survey provides an opportunity to more accurately assess the loads, for instance the lighting requirement, where the system is to be installed in an existing building. If possible ask people about the use to which each room is put and times during which it is occupied.
It is possible that there may be existing electrical wiring, for example if a generator is used. It may be possible to use all or part of this for the solar application. If this is the intention, then inspect the wiring and record its configuration.
4.3.5 Cabling
Take the opportunity to consider likely routes for the cabling, especially the heavy cables running from the controller to the solar array and the batteries. Measure the approximate length of these cables so that they can be correctly sized.