Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)

Scoping the Project

As with any project, before you start, you need to know what you want to achieve. In fact, it is one of the most important parts of the whole project. Get it wrong and you will end up with a system that will not do what you need it to.

It is usually best to keep your scope simple to start with. You can then flesh it out with more detail later.

Here are some examples of a suitable scope:

· To power a light and a burglar alarm in a shed on an allotment

· To provide power for lighting, a kettle, a radio and some handheld power tools in a workshop that has no conventional electrical connection

· To provide enough power for lighting, refrigeration and a TV in a holiday caravan

· To provide lighting and power to run four laptop computers and the telephone system in an office during a power cut

· To charge up an electric bike between uses

· To provide an off-grid holiday home with its entire electricity requirements

· To reduce my carbon footprint by producing electricity for all my home requirements

· To run an electric car entirely on solar energy

From your scope, you can start fleshing this out to provide some initial estimates on power requirements.

As mentioned in the previous chapter, I have created two online Solar Project Analysis tools, one for grid-tie systems and one for off-grid systems. You can find both of these tools on my website www.SolarElectricityHandbook.com. You will still need to collect the basic information to work with, but all the hard work is done for you. This tool will produce a complete project scope, work out the likely performance of your solar energy system and provide some ballpark cost estimates.

For the purpose of these next few chapters, I am going to use the example of providing a small off-grid holiday home with its entire electricity requirements.

This is a big project. In reality if you have little or no experience of solar electric systems or household electrics you would be best starting with something smaller. Going completely off-grid is an ambitious project, but for the purposes of teaching solar electric system design, it is a perfect example: it requires a detailed design that covers all of the aspects of designing a solar energy system.

Designing grid-tie or grid fallback systems

For our sample project, grid-tie is not an option as we are using solar power as an alternative to connecting our site to the electricity grid.

However, grid-tie is becoming a popular option, especially in the southern states of the United States and in European countries like Spain, Germany and the United Kingdom where generous government subsidies and feed-in tariffs are available.

In terms of scoping the project, it makes little difference whether you are planning a grid-tie or grid fallback system or not: the steps you need to go through are the same. The only exception, of course, is that you do not need to take into account battery efficiencies with grid-tie.

The biggest difference with a grid-tie or grid fallback system is that you do not have to rely on your solar system providing you with all your electricity requirements: you will not be plunged into darkness if you use more electricity than you generate.

This means that you can start with a small grid-tie or grid fallback system and expand it later on as funds allow.

Despite that, it is still a good idea to go through a power analysis as part of the design. Even if you do not intend to produce all the power you need with solar, having a power analysis will allow you to benchmark your system and will help you size your grid-tie system if you aim to reduce your carbon footprint by providing the electricity companies with as much power as you buy back.

Most grid-tie systems are sized to provide more power than you need during the summer and less than you need during the winter. Over a period of a year, the aim is to generate as much power as you use, although on a month-by-month basis this may not always be the case.

Many solar companies claim that this then provides you with a ‘carbon neutral’ system: you are selling your excess power to the electricity companies and then buying the same amount of electricity back when you need it.

If this is what you are planning to do with your grid-tie system, your scope is much simpler. You need to get your electricity bills for the past year and make a note of how much electricity you have used over the year. Then divide this figure by the number of days in the year to work out a daily energy usage and ensure your system generates this as an average over the period of a year.

Because you are not generating enough electricity during the winter months in a carbon neutral grid-tie system, you need fewer solar panels than you would need to create an entirely stand-alone system.

Comparing supply with demand

If you are designing a grid-tie system, it can be interesting to compare the supply of solar energy with your electricity usage pattern. By comparing supply with demand, you can see how closely solar energy production matches your own usage and this, in turn, can be used as an indicator to identify how environmentally beneficial solar energy is for you.

To do this, you will need to ascertain your monthly electricity usage for each month of the year. Your electricity supplier may already provide you with this information on your electricity bill. If not, you should be able to request this from them.

Once you have this information, visit www.SolarElectricityHandbook.com and fill in the Grid-Tie Solar Project Analysis, including your individual monthly consumption figures. In the report that is e-mailed to you, you will see a chart that allows you to see how closely your electricity usage maps onto solar energy production.

This report will also provide you with an approximate estimate for the carbon footprint for each kilowatt-hour of electricity you produce from your solar array, based on the production and installation of your solar array and the likely amount of energy that it will generate during its lifetime.

Based on this, it is possible to see whether installing solar energy is likely to produce real-world environmental savings.

Fleshing out the scope

Now we know the outline scope of our project, we need to quantify exactly what we need to achieve and work out some estimates for energy consumption.

Our holiday home is a small two-bedroom cottage with a solid fuel cooker and boiler. The cost of connecting the cottage to the grid is £4,500 (around $7,200) and I suspect that solar electric power could work out significantly cheaper.

The cottage is mainly used in the spring, summer and autumn, with only a few weekend visits during the winter.

Electricity is required for lighting in each room, plus a courtesy light in the porch, a fridge in the kitchen and a small television in the sitting room. There also needs to be surplus electricity for charging up a mobile phone or MP3 player and for the occasional use of a laptop computer.

Now we have decided what devices we need to power, we need to find out how much energy each device needs, and estimate the daily usage of each item.

In order to keep efficiency high and costs low, we are going to work with low-voltage electrics wherever possible. The benefits of using low-volt devices rather than higher grid-voltage are twofold:

· We are not losing efficiency by converting low-volt DC electrics to grid-voltage AC electrics through an inverter.

· Many electronic devices that plug into a grid-voltage socket require a transformer to reduce the power back down to a low DC current, thereby creating a second level of inefficiency

Many household devices, like smaller televisions, music systems, computer games consoles and laptop computers, have external transformers. It is possible to buy transformers that run on 12-volt or 24-volt electrics rather than the AC voltages we get from grid power, and using these is the most efficient way of providing low-voltage power to these devices.

There can be disadvantages of low-voltage configurations, however, and they are not the right approach for every project:

· If running everything at 12-24 volts requires a significant amount of additional rewiring, the cost of carrying out the rewiring can be much higher than the cost of an inverter and a slightly larger solar array

· If the cable running between your batteries and your devices is too long, you will get greater power losses through the cable at lower voltages than you will at higher voltages

If you already have wiring in place to work at grid-level voltages, it is often more appropriate to run a system at grid voltage using an inverter, rather than running the whole system at low voltage. If you have no wiring in place, running the system at 12 or 24 volts is often more suitable.

Producing a power analysis

The next step is to investigate your power requirements by carrying out a power analysis, where you measure your power consumption in watt-hours.

You can find out the wattage of household appliances in one of four ways:

· Check the rear of the appliance, or on the power supply

· Check the product manual

· Measure the watts using a watt meter

· Find a ballpark figure for similar items

Often a power supply will show an output current in amps rather than the number of watts the device consumes. If the power supply also shows the output voltage, you can work out the wattage by multiplying the voltage by the current (amps):

Power (watts) = Volts x Current (amps)

P = V x I

For example, if you have a mobile phone charger that uses 1.2 amps at 5 volts, you can multiply 1.2 amps by 5 volts to work out the number of watts: in this example, it equals 6 watts of power. If I plugged this charger in for one hour, I would use 6 watt-hours of energy.

A watt meter is a useful tool for measuring the energy requirements of any device that runs on high-voltage AC power from the grid. The watt meter plugs into the wall socket and the appliance plugs into the watt meter. An LCD display on the watt meter then displays the amount of power the device is using. This is the most accurate way of measuring your true power consumption.

Finding a ballpark figure for similar devices is the least accurate way of finding out the power requirement and should only be done as a last resort. A list of power ratings for some common household appliances is included in Appendix C.

Once you have a list of the power requirements for each electrical device, draw up a table listing each device, whether the device uses 12-volt or grid voltage, and the power requirement in watts.

Then put an estimate in hours for how long you will use each device each day and multiply the watts by hours to create a total watt-hour energy requirement for each item.

You should also factor in any ‘phantom loads’ on the system. A phantom load is the name given to devices that use power even when they are switched off. Televisions in standby mode are one such example, but any device that has a power supply built into the plug also has a phantom load. These items should be unplugged or switched off at the switch when not in use. However, you may wish to factor in a small amount of power for items in standby mode, to take into account the times you forget to switch something off.

If you have a gas-powered central heating system, remember that most central heating systems have an electric pump and the central heating controller will require electricity as well. A typical central heating pump uses around 60 watts of power a day, whilst a central heating controller can use between 2 and 24 watts a day.

Once complete, your power analysis will look like this:

A word of warning

In the headlong enthusiasm for implementing a solar electric system, it is very easy to underestimate the amount of electricity you need at this stage.

To be sure that you do not leave something out which you regret later, I suggest you have a break at this point. Then return and review your power analysis.

It can help to show this list to somebody else in order to get their input as well. It is very easy to get emotionally involved in your solar project, and having a second pair of eyes can make a world of difference later on.

When you are ready to proceed

We now know exactly how much energy we need to store in order to provide one day of power. For our holiday home example, that equates to 662 watt-hours per day.

There is one more thing to take into account: the efficiency of the overall system.

Batteries, inverters and resistance in the circuits all reduce the efficiency of our solar electric system. We must consider these inefficiencies and add them to our power analysis.

Calculating inefficiencies

Batteries do not return 100% of the energy used to charge them. The Charge Cycle Efficiency of the battery measures the percentage of energy available from the battery compared to the amount of energy used to charge it.

Charge cycle efficiency is not a fixed figure, as the efficiency can vary depending on how quickly you charge and discharge the battery. However, most solar applications do not overstress batteries and so the standard charge cycle efficiency figures are usually sufficient.

Approximate charge cycle efficiency figures are normally available from the battery manufacturers. However, for industrial quality ‘traction’ batteries, you can assume 95% efficiency, whilst gel batteries and leisure batteries are usually in the region of 90%.

If you are using an inverter in your system, you need to factor in the inefficiencies of the inverter. Again, the actual figures should be available from the manufacturer but typically, you will find that an inverter is around 90% efficient.

Adding the inefficiencies to our power analysis

In our holiday home example, there is no inverter. If there were, we would need to add 10% for inverter inefficiencies for every grid-powered device.

We are using batteries. We need to add 5% to the total energy requirement to take charge cycle efficiency into account.

5% of 662 equals 33 watts. Add this to our power analysis, and our total watt-hour requirement becomes 695 watt-hours per day.

When do you need to use the solar system?

It is important to work out at what times of year you will be using your solar electric system most. For instance, if you are planning to use your system full time during the depths of winter, your solar electric system needs to be able to provide all your electricity even during the dull days of winter.

A holiday home is often in regular use during the spring, summer and autumn, but left empty for periods of time during the winter.

This means that, during winter, we do not need our solar electric system to provide enough electricity for full occupancy. We need enough capacity in the batteries to provide enough electricity for, say, the occasional long weekend. The solar array can then recharge the batteries again, once the home is empty.

We might also decide that, if we needed additional electricity in winter, we could have a small standby generator on hand to give the batteries a boost charge.

For the purposes of our holiday home, our system must provide enough electricity for full occupancy from March to October and occasional weekend use from November until February.

Keeping it simple

You have seen what needs to be taken into account when creating a power analysis and calculating the inefficiencies. If you are planning to use the online tools to help you, now is the time to use them.

Visit www.SolarElectricityHandbook.com and follow the links to either the Off-Grid or Grid-Tied Solar Project Analysis tools, which can be found in the Online Calculators section. This will allow you to enter your devices on the power analysis, select the months you want your system to work and select your location from a worldwide list. The system will automatically e-mail you a detailed solar analysis report with all the calculations worked out for you.

Improving the scope

Based on the work done, it is time to put more detail on our original scope. Originally, our scope looked like this:

Provide an off-grid holiday home with its entire electricity requirements.

Now the improved scope has become:

Provide an off-grid holiday home with its entire electricity requirements, providing power for lighting, refrigeration, TV, laptop computer and various sundries, which equals 695 watt-hours of electricity consumption per day.

The system must provide enough power for occupation from March until October, plus occasional weekend use during the winter.

There is now a focus for the project. We know what we need to achieve for a solar electric system to work. Now we need to go to the site and see if what we want to do is achievable.

In conclusion

· Getting the scope right is important for the whole project

· Start by keeping it simple and then flesh it out by calculating the energy requirements for all the devices you need to power

· If you are designing a grid-tie system, you do not need to go into so much detail: you can get the usage information from your electricity company. It is probably included on your electricity bill

· If you are designing a grid-tie system, you can make a reasonable estimate of its environmental benefit by comparing solar energy supply with your demand on a month-by-month basis

· Do not forget to factor in ‘phantom loads’

· Because solar electric systems run at low voltages, running your devices at low voltage is more efficient than inverting the voltage to grid levels first

· Thanks to the popularity of caravans and boats, there is a large selection of 12-volt appliances available. If you are planning a stand-alone or grid fallback system, you may wish to use these in your solar electric system rather than less efficient grid-voltage appliances

· Even if you are planning a grid-tie system, it is still useful to carry out a detailed power analysis

· Do not forget to factor in inefficiencies for batteries and inverters

· Take into account the times of year that you need to use your solar electric system

· Once you have completed this stage, you will know what the project needs to achieve in order to be successful