Off grid load control
When you are off grid and rely on your own stand alone power system, a typical scenario is that you can do a lot more during the day and when it is night time you need to be a lot more careful as all of your energy comes out of a battery bank. Typically there will be a backup generator but it would be undesirable to have this become a major contributor to energy, rather it should be a backup and contribute when the weather is poor.
Off grid systems require a lot of consideration and design work in order to work well. Usually the biggest design parameter is customer budget and this plays a big part on overall performance. Many times a good design cannot be reached as the budget is not where it should be and the load requirements need to be revisited. The big factor is the amount of energy storage (battery bank size and type). Batteries do not create energy, they only store it during the day for use at night time. There is no government incentive to install batteries and when you are on the grid they certainly do not pay for themselves like solar panels do. Off the grid you have no choice but to have batteries.
How do we improve this financial situation? As users, we need to be smarter about when we use energy. As solar designers and manufacturers we need to implement smarter load control. The normal situation with an off grid solar system is the battery is at a mid to low level in the morning and all of the energy from the sun will go to charging the battery. A well designed system will have this battery full at some point during the morning on a typical day. It is not too hard to achieve this as the rebate that helps pay for a system is based on the solar panels, not the batteries. It is therefore desirable to put as much solar as you can on the roof. Getting the battery bank full quickly is a great thing, but what happens when it is completely full?
Batteries are like a fixed size container. Once it is full it can take no more. At this point an off grid system will need to throttle down the solar power to a level that will just run the loads in the house. At this point the sun will be high in the sky and we have the potential to generate lots of electricity but it is going to waste. This is where we need to be smart. Smart people live off the grid, but people who live off the grid need to be smart. There are so many uses for this electricity during the day and to be smart we need to use it so that we do not need to use the battery energy at night. Typical examples are air conditioning, water heating, pool filtering, irrigation, floor heating, laundry, dish washing and many more. As a user of the off grid system we need to make our habits suit the best performance and make decisions when we use energy.
When the battery bank is full and we then turn on a load, the solar will then “throttle up” to meet that demand. This is assuming there is enough sunshine at the time. If the sun is not quite enough the difference will come out of the battery. The battery will be there to cover the shortfall, but we need to be careful not rely on this too much. Think of it as a way of smoothing over when a small cloud gets in the way of the sun. The holy grail in off grid is harnessing the potential of the sun after the battery is full and keeping the battery full. This means reacting to conditions automatically. There are certain loads that are suitable to “soak up” this excess energy. A good one is water heating. Basic electric hot water heaters are just like a kettle and do not mind turning off and on many times a day. Floor heating is a good one, but we really only need this in winter time and as it is a form of space heating it will be energy hungry and winter is when we will generate the least, but we may as well use it as it will cut down on other space heating sources which will inevitably be fuels. A couple of bathroom floor heating elements or heated towel rails can be a good load. Air conditioning is a great load in summer time. It goes hand in hand with solar. We need it when there is a lot of energy available so it makes perfect sense. Air conditioning is not the sort of load that we can rapidly turn on an off as it has compressors, fans and electronics that will not like the power cycling, but if we consider it an all day “base load” it will give you a cool home all day, every day and this will give you a better chance of having a comfortable home at night as all the building materials have retained a low level of heat during the day. We can design a system that first charges a battery and once it is near to full we can implement priority loads. Our air conditioning can be our base load and our water heating can be our “reactive load” which we can turn on and off for short periods depending on what is available.
The solar production curve
If we plot our power output on a curve with the power on the y axis and the time on the x axis we get a curve that tells a story for the day. The area under this curve is the total energy for the day. When the curve is smooth and does not have dips, it is the maximum that can be done on any particular day. This will be a day with no clouds in the sky. The area under the curve is at the maximum it possibly can be for that day of the year. The issue is that we can look at a curve for a day in the past and make load choices to extract this power very well, but looking into the future we really do not know what the curve will look like. The weather forecast can give us an idea, but we do not know in the next 10 minutes if a little cloud will cover the sun and drop the power output to 40% and cause precious energy to come out of the battery to cover the shortfall.
Our base loads should be able to stay on, but if we have some reactive loads like water heating or resistive space heating we can turn them off until the cloud moves out of the way. To do this we need a way of measuring irradiance. For this we use a pyranometer. A simple little solar panel with a load resistor can be used to input a voltage to the analgue input on a Selectronic inverter. This input can be used to program a relay which can control a load. With a little bit of testing we can set it to make sure the load is not on until the pyranomteer senses there will be enough electricity to power that load.
The Selectronic inverter is the best off grid inverter in the world. It has so many features and I have been able to implement many features to control loads smartly. It has 4 relay outputs and many digital and analogue inputs. It also has its own measured values that can be used as an input. You can program a relay to function at a certain state of charge, or a certain battery voltage or at a certain load level. Here’s where computer programming experience helps. Many inverters and even charge controllers have relays and programmable functions. There are also third party devices or even basic time clocks that can be used to control loads. The Selectronic inverter has the best functions that I know of and I am very hopeful there will be improvements made to their software to better harness the hardware potential for load control.
Case Study - Floor and water heating off grid RYLSTONE
Here is a system that was installed in 2019. It is quite a large system with the brief of harnessing the excess energy available in winter time to heat the floor. The idea is that although there is a high installation cost, there will be no ongoing costs. Additionally, as the system uses Fronius grid connect inverters which are designed to work at the maximum all day long without being throttled, they won't mind doing the same off the grid in the winter time.
System specs:
20kW PV on the roof
15kW PV inverters
7.5kW battery inverter
Load control relay enclosure
Load control heating distribution board
Commercial Rheem 315L hot water heater with 3 bottom elements
Domestic Rheem 315L dual element water heater for water consumption
Circulating pump
Time clock
Under floor piping, manifold and temperature sensors
This is quite a large system and we are asking the most from this system at the time of year that it will deliver the least. For the design the most important thing was to make the solar on the roof as big as possible. In this case we are limited by the usable roof space and system components maximums.
After the first year of operation the reports have been excellent. There were some days in the middle of the year that the system struggled due to very poor weather and these days are the ones when you can do with the most heating, but generally the winter in the central west is characterised by fine weather and beautiful sunny winter days. In the most part it heated half of a large 4 bedroom house very well. With the manifold, heating circuits could be turned off to concentrate the heat into priority areas. The less water circulating the higher the temperature.
The real advantages of this system is that is has zero ongoing costs to run. It also is easy to run as it is automatic and runs all the time. If you leave the home for a week it will keep running, and when you return you will have lovely warm floors to welcome you. There are no complex machines to wear out requiring maintenance and no noise. For this system there has been talk of adding a fuel burner to the system to top up what the solar is unable to deliver the full requirements. A small LPG burner could be a good addition to heat the whole house when guests are staying. The solar floor heating would then be the baseline and the fuel burner the booster.
This system has been a great success. We have, in the most part harnessed the available energy throughout the whole of winter at no ongoing costs. This has reduced by a long way the need for fuel burning. The system ran until mid November then was throttled down to just domestic water heater for the summer. When the system was at full pace is would turn 90 - 100kWh of electrical energy into heat energy in a day. This is about $30 worth of electricity per day. The customer is happy now as it will deliver this beautiful, silent heating to the home for many years to come absolutely free.
Smart Appliances
There is a lot of development to come in this area, both on the grid and off the grid. One development is DRM - Demand Response Modes. This are now found in air conditioners and similar appliances like heat pumps and electric car chargers. These can be thought of as signals that tell the load how much it can draw. DRM has been implemented due to challenges on the electricity grid. The idea is when the weather gets too hot and everyone has their air conditioning on, the grid operators can send these signals to these loads to throttle down in order to prevent blackouts. I don’t know about you, but I wouldn’t be happy about that at all. Thankfully we do not need to worry about that when we are off the grid, but these modes can become useful to an off grid system designer.
I have used these DRM modes on air conditioners off the grid a few times. These are terminals in the outdoor unit of the air conditioner so labeled. All that you need to do it close the contact between C (common) and the appropriate DRM terminal and the air conditioner can react accordingly. This is another great way of using the Selectronic relays to control the load. The modes I have used successfully are:
DRM 1 - Do not consume power / compressor off
DRM 2 - run at 50%
Generally off the grid we don’t need to worry about the air conditioning except the warm nights. By using the DRM function we can automate the air conditioning to throttle down first and then turn the compressor off later in the night. The air conditioner retains power and does not abruptly get turned off. In summer the fan can still run which draws minimum power. This is a safety net to prevent an air conditioner flattening a battery bank. In the morning when the battery level rises the air conditioner will first run at 50% then a little later at 100%. All that is required is a signal cable from the Inverter to the condenser unit outside
Figure 5 - An air conditioner off grid with DRM enabled. The air conditioner is off until the battery is sufficiently charged. Then the mode changed from DRM 1 to DRM 2 allowing running at 50%, shortly after all DRM is inactive allowing full running all day long. After dark the air conditioner steps back down again gradually and off completely late at night.
The Future
In the future there will be more smart appliances and better integration between the appliances and the generating plants. Data is the key. Having excellent monitoring of what is occurring and excellent software to control devices is the way to success. Much of what you have read is experimental, particularly the case study in Rylstone. It uses old fashioned relay and contactor logic. We have been able to use existing controls to create the system and we have had the confidence to implement it because we have excellent monitoring. Nothing is unrecorded and we are learning all the time and experimenting with new technology and putting pressure on the manufacturers to improve their products. We are hoping to implement finer control in future for our off grid floor heating case study by better utilising the pyranometer control and a requested improvement from Selectronic.
All this is worth doing because the energy from solar panels is free and the more we harness this energy with smart methods, the less we need to harness from batteries at night time. This will put downward pressure on off grid system prices and give you more for your money.
Very soon we will be reviewing an off grid air conditioner. It uses solar panels to directly power the appliances. No inverter, no battery. Stay tuned!
Paul Deegan