Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)
The Four Configurations for Solar Power
There are four different configurations you can choose from when creating a solar electricity installation. These are stand-alone (sometimes referred to as off-grid), grid-tie, grid-tie with power backup (also known as grid interactive) and grid fallback.
Here is a brief introduction to these different configurations:
Stand-alone/off-grid
Worldwide, stand-alone solar photovoltaic installations are the most popular type of solar installation there is. It is what solar photovoltaics were originally created for: to provide power at a location where there is no other source easily available.
Whether it is powering a shed light, providing power for a pocket calculator or powering a complete off-grid home, stand-alone systems fundamentally all work in the same way: the solar panel generates power, the energy is stored in a battery and then used as required.
In general, stand-alone systems are comparatively small systems, typically with a peak power generation of under one kilowatt.
Almost everyone can benefit from a stand-alone solar system for something, even if it is something as mundane as providing an outside light somewhere. Even if you are planning on something much bigger and grander, it is often a good idea to start with a very small and simple stand-alone system first. Learn the basics and then progress from there.
Examples of simple stand-alone systems
The vending machine
ByBox is a manufacturer of electronic lockers. These are typically used for left luggage at railway stations or at airports, or situated at shopping malls or fuel stations and used as part of a delivery service for people to collect internet deliveries, so they do not need to wait at home.
One of the biggest issues with electronic lockers has often been finding suitable locations to place them where a power source is available. ByBox overcame this issue by building an electronic locker with a solar roof to provide permanent power to the locker.
The solar roof provides power to a set of batteries inside the locker. When not in use, the locker itself is in standby mode, thereby consuming minimal power. When a customer wishes to use the locker, they press the START button and use the locker as normal.
The benefit to ByBox has been twofold: they can install a locker bank in any location, without any dependence on a power supply. Secondly, the cost of the solar panels and controllers is often less than the cost of installing a separate electricity supply, even if there is one nearby.
Recreational vehicles
Holidaying with recreational vehicles or caravans is on the increase, and solar energy is changing the way people are going on holiday.
In the past, most RV owners elected to stay on larger sites, which provided access to electricity and other facilities. As recreational vehicles themselves become more luxurious, however, people are now choosing to travel to more remote locations and live entirely ‘off-grid’, using solar energy to provide electricity wherever they happen to be. Solar is being used to provide all the comforts of home, whilst offering holidaymakers the freedom to stay wherever they want.
Grid-tie
Grid-tie is gaining popularity in Europe and the United States. This is due to the availability of grants to reduce the installation costs and the ability to earn money by selling electricity back into the electricity companies through a feed-in tariff.
Feed-in tariff schemes vary around the world and are not available everywhere. Where they exist, your local electricity company buys electricity from solar producers at an agreed rate per kilowatt-hour. In some countries, this price has been set at an inflated rate by government in order to encourage people to install solar. In other countries and regions, the price is agreed by the electricity companies themselves.
In a grid-tie system, your home runs on solar power during the day. Any surplus energy that you produce is then fed into the grid. In the evenings and at night, when your solar energy system is not producing electricity, you then buy your power from the electricity companies in the usual way.
The benefit of grid-tie solar installations is that they reduce your reliance on the big electricity companies and ensure that more of your electricity is produced in an environmentally efficient way.
One disadvantage of most grid-tie systems is that if there is a power cut, power from your solar array is also cut.
Grid-tie can work especially well in hot, sunny climates, where peak demand for electricity from the grid often coincides with the sun shining, thanks to the high power demand of air conditioning units. Grid-tie also works well where the owners use most of the power themselves.
An example of a grid-tie system
Si Gelatos is a small Florida-based ice-cream manufacturer. In 2007, they installed solar panels on the roof of their factory to provide power and offset some of the energy used in running their cold storage facility.
“Running industrial freezers is extremely expensive and consumes a lot of power,” explains Dan Foster of Si Gelatos. “Realistically, we could not hope to generate all of the power from solar, but we felt it was important to reduce our overall power demand and solar allowed us to do that.”
Cold storage facilities consume most of their power during the day in the summer, when solar is running at its peak. Since installing solar power, Si Gelatos has seen its overall energy consumption drop by 40% and now hardly takes any power from the utilities during peak operating times.
“Solar has done three things for our business,” says Dan. “Firstly, it is a very visible sign for our staff that we are serious about the environment. This in turn has made our employees more aware that they need to do their bit by making sure lights and equipment are switched off when they are not needed. Secondly, it shows our customers that we care for the environment, which has definitely been good for goodwill and sales. Thirdly, and most importantly, we’re genuinely making a real contribution to the environment, by reducing our electricity demand at the time of day when everyone else’s demand for electricity is high as well.”
Grid-tie with power backup (grid interactive)
Grid-tie with power backup - also known as a grid interactive system - combines a grid-tie installation with a bank of batteries.
As with grid-tie, the concept is that you use power from your solar array when the sun shines and sell the surplus to the power companies. Unlike a standard grid-tie system, however, a battery bank provides contingency for power cuts so that you can continue to use power from your system.
Typically, you would set up ‘protected circuits’ within your building that will continue to receive power during a power outage. This ensures that essential power remains available for running lights, refrigeration and heating controllers, for example, whilst backup power is not wasted on inessential items such as televisions and radios.
If there is a potential for main power to be lost for several days, it is also possible to design a system to incorporate other power generators into a grid interactive system, such as a generator. This would allow a grid interactive system to work as a highly efficient uninterruptable power supply (UPS) for extended periods of time.
The cost of a grid-tie system with power backup is higher than a standard grid-tie system, because of the additional cost of batteries and battery controllers. Typically, having power backup will add 12-20% of additional costs over a standard grid-tie system.
As with normal grid-tie systems, it is possible to sell surplus power back to the utility companies in some countries, allowing you to earn an income from your solar energy system.
An example of a grid interactive system
Grid interactive systems are gaining popularity with rural farms in the United Kingdom, where even short power blackouts can cause significant disruption.
Traditionally, farms have countered this by using generators to provide light and power. However, between 2009 and 2011, when the UK Government were offering large incentives for installing solar power, many farmers fitted grid interactive systems onto their buildings, providing themselves with an income by selling electricity to the electricity utility companies and giving themselves backup power in case of a power blackout.
The additional cost of installing a grid interactive system over a standard grid-tie system is more than offset by the low running costs and ease of use of the system. Farmers do not need to buy and run generators and the system is almost entirely maintenance-free. This is a big contrast with generator systems, which need to be tested and run regularly in order to ensure they are working effectively.
Grid fallback
Grid fallback is a lesser-known system that makes a lot of sense for smaller household solar power systems. For most household solar installations where solar is being installed for technical or environmental reasons, grid fallback is my preferred solution. Operationally it is effective, it is cost-effective and it is environmentally extremely efficient.
With a grid fallback system, the solar array generates power, which in turn charges a battery bank. Energy is taken from the battery and run through an inverter to power one or more circuits from the distribution panel in the house.
When the batteries run flat, the system automatically switches back to the grid power supply. The solar array then recharges the batteries and the system switches back to solar power.
With a grid fallback system, you do not sell electricity back to the electricity companies. All the power that you generate, you use yourself. This means that some of the grants that are available for solar installations in some countries may not be available to you. It also means that you cannot benefit from selling your electricity back to the electricity companies.
For this reason, grid fallback makes more sense in countries where there is no feed-in tariff available, such as India, or in countries like Australia that have financial incentives available for both grid-tied and off-grid systems.
Grid fallback systems provide most of the benefits of a grid interactive system, with the additional benefit that you use your own power when you need it, rather than when the sun is shining. This reduces your reliance on external electricity supplies during peak load periods, which ensures that your system has an overall environmental benefit.
The other significant benefit of a grid fallback system is cost: you can genuinely build a useful grid fallback system to power one or more circuits within a house for a very small investment and expand it as budget allows. I have seen grid fallback systems installed for under £400 ($680), providing a useful amount of power for a home. In comparison, even a very modest grid-tie system costs several thousands of pounds.
There is a crossover point where a grid-tie system works out more cost-effective than a grid fallback system. At present, that crossover point is around the 1kWh mark: if your system is capable of generating more than 1kW of electricity per hour, a grid-tie system may be more cost-effective. If your system generates less than 1kW of electricity per hour, a grid fallback system is almost certainly cheaper.
Unless you are looking to invest a significant amount of money on a larger grid-tie system in order to produce more than 1 kW of power per hour, or if you want to take advantage of feed-in tariffs, a grid fallback solution is certainly worth investigating as an alternative.
An example of a grid fallback system
Back in 2001, Colin Metcalfe installed a solar panel onto the roof of his garage, in order to charge an old car battery, which in turn powered a single light and a small inverter. After a power cut that winter, Colin decided to expand his system in order to provide basic power to his house.
“I wanted to ensure I always had enough power in my home to power lights and to ensure my heating system would work,” explained Colin. “I have gas heating, but the controllers are all electric, which means that if there is a power cut, I have no heating at all. In addition, I liked the idea of free electricity that was generated in an environmentally friendly way.”
Colin upgraded his system bit by bit, as funds allowed. “An electrician fitted a new distribution panel (consumer unit) for my essential circuits, and this was connected up to the main panel via an automatic transfer switch. Then I added additional solar panels and batteries over the years as I could afford them.”
This automatic transfer switch meant the essential circuits would receive power from the solar array or the batteries while power was available, but switch back to utility power when the batteries ran flat. Originally, the system provided around half the power he needed, but as he has added to the system, more and more of his power now comes from his solar array. “Today I have around 1.4kW of solar panels on the roof of my garage,” says Colin. “They look a bit odd as no two panels are alike, as I have bought them bit by bit as funds allow, but they now provide all the power I need around the year for all my essential circuits.”
Grid failover
Alternatively, you can configure a grid fallback system as a grid failover system.
A grid failover system kicks in when there is a power failure from your main electricity supply. In effect, it is an uninterruptable power supply, generating its power from solar energy.
The benefit of this configuration is that if you have a power cut, you have contingency power. The disadvantage of this configuration is that you are not using solar power for your day-to-day use.
Although rare in Europe and America, grid failover systems used to be more common in countries where power failures are commonplace. In Africa and in many parts of Asia, grid failover systems reduce the reliance on power generators for lighting and basic electricity needs.
However, in most cases, customers have found that a grid fallback or grid interactive system is more suitable for their needs. I am aware of two grid failover systems that have been installed in the past: both of these have since been reconfigured as grid fallback systems.
How grid-tie systems differ from stand-alone
Generally, stand-alone and smaller grid fallback systems run at low voltages, typically between 12 and 48 volts. This is because batteries are low-voltage units and so building a stand-alone system at a low voltage is a simple, flexible and safe approach.
Grid-tie systems tend to be larger installations, often generating several kilowatts of electricity each hour. As the electricity is required as a high-voltage supply, it is more efficient to connect multiple solar panels together to produce a high voltage circuit, rather than use an inverter to step up the voltage. This high-voltage DC power is then converted into an AC current by a suitable grid-tie inverter.
Grid-tie systems either link multiple solar panels together to produce a solar array voltage of several hundred volts before running to the inverter, or have a small inverter connected to each solar panel to create a high-voltage AC supply from each panel.
The benefit of this high voltage is efficiency. There is less power loss running high-voltage, low-current electricity through cables from the solar array.
For stand-alone battery-based systems, low-voltage is the best solution, as the battery banks tend to work better as low-voltage energy stores. For grid-tie systems where the energy is not being stored in a battery bank, the higher-voltage systems are the best solution. Neither approach is inherently ‘better’: it all depends on the type of system you are designing.
In conclusion
· Solar can be used in a number of different ways and for many different applications
· Stand-alone systems are the simplest and easiest to understand. They tend to be comparatively small systems, providing power where no other power source is easily available
· With grid-tie, your solar energy system generates electricity that is then used normally. Any excess electricity production is exported onto the grid
· Grid-tie with power backup (also known as grid interactive) provides you with the benefits of a grid-tie system with the added benefit that power remains available even if electricity to your area is cut off
· Grid fallback systems have more in common with stand-alone systems than grid-tie systems. In design they are very similar to stand-alone systems, with an inverter running from a bank of batteries and an automatic transfer switch to switch power between the solar energy system and the grid power supply
· Grid failover systems are comparatively rare now, but provide uninterruptable power supplies using solar as the backup source
· Grid-tie systems have a different design to stand-alone systems. They tend to be high-voltage systems, whereas stand-alone systems run at much lower voltages