Converting solar energy to useful electricity - Grid Tie Inverters and solar geysers

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A really impressive technology is developing at present relating to the useful conversions of solar energy.

Solar panels are becoming quite cheap and have a long operating life (20 years if well made). However they are difficult to interface to harness the available power. A typical panel might have an open circuit voltage of 22 volts, a short circuit current of 3 amps and have an optimum power transfer point elsewhere say at 15 volts. In addition a cloud passing across the direct sun path might cause the output power to drop to just 10% of the energy compared to the energy without the cloud. All this means that one is getting continually fluctuating energy from the panels and it is very difficult to design for a steady load- especially something using a motor.

In the past one might have stored the energy in some form of battery, but this gives very little of the potential energy available from the solar panel to a useful load. Batteries are expensive, difficult to charge and discharge quickly, have small capacities and a limited life.

The ideal situation is to immediately convert the solar energy to mains electricity that is compatible with the supplied mains and to use the energy in the normal AC load of the building, reducing the amount of energy drawn from the municipal supply.

You cannot use any inverter to convert the DC voltage from the solar panel to AC mains compatible energy. It has to be generated in exact phase and frequency with the incoming mains otherwise it is going to be vaporised.

The type of inverter needed is called a Grid-Tie inverter.

It is wired directly onto the mains supply and the solar panel provides the energy. The inverter continually monitors the solar panel and as soon as there is sufficient energy it starts to monitor the AC mains supply determining the frequency and the phase angles.

It starts its inverter at the same frequency as the incoming mains supply but at a low output voltage and gets the two voltages in exact phase with each other. It then increases the output voltage to start supplying in phase electricity and using up the available solar power.

On the solar input side it adjusts the load voltage to get maximum power transfer from the solar panel by monitoring the DC voltage and current to operate at the Maximum Power transfer point. It continually adjusts these values so that it can handle variations in the available solar power such as might happen with a cloud passing through the direct sun path.

The inverter has another feature called islanding, which shuts down the entire process in the event of an incoming mains failure.

Grid-Tie inverters have been around for a long time for major solar and wind turbine installations. They have however been quite expensive (US$1500 for a 15KVA system)

The new development is in the form of micro grid-tie inverters which allow simple systems of 300 watts or 500 watts to be implemented in modular form. These are not wired into the main switch board of the establishment, but plug into a normal AC outlet in a room. They are also cascadable allowing many 300 watt or 500 watt units to work in parallel so that larger loads can be addressed. They cost only about US$150 each.

The performance advantage of the micro grid tie inverter compared to the historic large scale grid tie inverter is that it can work with a single solar panel as it is designed to work with 12 volt, 24 volt or 48 volt DC systems. The advantage with lower voltage small systems is that they are safer for the DIY enthusiast, you can start your system with a small working system and expand as your needs and finances grow, and that they are less susceptible to loss of performance caused by shadows. On traditional grid tie systems where a single inverter is used which is fed from many panels, the panels are wired in series to supply DC voltages as high as 400 volts. As soon as a shadow falls on any one panel, the performance of the entire system is compromised as all the panels are in series. With the microgrid tie inverter a shadow on one panel affects only that inverter while all the others in the network operate at full capacity. There are market developments that in future it is likely that the grid tie inverter will be built into each solar panel and that the output of the panel will be at mains voltage with all panels tied in parallel.

Another advantage of the micro grid tie inverter is that it will optimally transfer power from its solar panel to the mains network. As solar panels made by different manufacturers have different performance specifications, the use of micro grid tie inverters allows a network of panels with different specifications to work optimally.

We have six of these systems running at present which generate about 70% of the electricity used during daytime. We have power meters on the incoming mains power supplied from the municipality and on the power supplied from the solar panels and continuously monitor the effectiveness of the system. When clouds pass over the setup one instantly observes the change in output from the solar network and the increase from the municipal network to keep the building load running optimally.

What happens when you are generating more solar energy than you can use and energy is being returned to the municipal grid?
Traditionally municipal meters were of the magnetic disc type. With these meters if you generate more energy than you use, the meter is mechanical and it will run backwards crediting you for the extra energy you returned which will be returned when you are not over generating - thereby ensuring you benefit from your extra contribution.

With Prepaid electricity systems and remote electricity meter monitoring, the municipal meter is becoming an electronic device. In computing your power usage, it takes into account the magnitude of the voltage and the magnitude of the current, and the phase angle between the two components so that it can calculate the true power supplied. When you overgenerate, current flows back to the municipality and the meter detects this as the phase angle between the voltage and current reverses. The meters do not give you credit for this returned power, but rather calculate that no power is being supplied by the municipality.

Above: A meter measuring the incoming current from the solar panels to one of the micro grid-tie inverters.

Power meters monitoring the supply of energy. The meter on the left shows incoming power to the building which is being supplied by the solar panels, while the one on the right shows supplementary power from the municpal supply. Units are kilowatt hours. We have as of July 2015 generated 3000kwH of electricity in our small system at (8-10)kwH/day for a full sun day and saved 3.0 tonnes of carbon release..

At current electricity rates, we expect payback within 5 years without any form of solar subsidy.

The following graph shows the spread of energy converted via the grid-tie inverters on a typical spring day in johannesburg, South Africa. Here we are 1700 meters above sea level and our winters are clear of cloud. Peak power occurs at 11 a.m.

Solar water heating - solar geysers

Heating water via a solar geyser is the most effective use of solar energy that the home owner can implement. The equipment is cheap, it is low tech and payback comes very quickly.

A low pressure solar water heater. These are very efficient in heating water. In this case the heating tubes are double walled tubes with the gap between the two tubes being evacuated. The water being heated is inside the inner tube and rises as it is heated to be replaced with colder water from the header tank. The vacuum insulation between the hot water and the outside is so effective that when you touch the tubes they feel cold. This one has a 100 litre header tank which achieves nearly 70 degrees celsius in one day sunshine. In our situation we feed this to the kitchen sink as a third feed and only switch on the electric geyser for the kitchen if there are a number of days of overcast weather.

A high pressure solar water system allows the use of balanced pressure taps for showers, baths and basins to also be used with solar systems. The pressure in the balanced water system operates at 400 kpA, while the solar heater is a low pressure system. Inside the solar storage tank is a coiled pipe through which the high pressure water flows. This pipe transfers heat by conduction from the heated solar water tank to the water flowing in the coiled pipe. The water in the coiled pipe is fed to the input water connection of the electric geyser meaning that instead of filling the geyser with cold municipal water, it is filled with preheated water meaning less electricity is needed to heat the water to operating temperature. When using the showers etc, the temperature of the solar geyser tank drops as heat is transferred out of the tank.

The formula for converting temperture rise to the equivalent electricity usage is
1 calorie of energy causes 1 cc of water to rise 1 degree centigrade
1 calorie of energy is equivalent to 4.17 joules of energy
1 joule = 1 watt-second
1 kw-hour of energy raises 100 liters of water 8.633 degrees centigrade
1 kw-hour of energy raises 200 liters of water 4.31 degrees centigrade

From the same calculation, every four degree drop in temperature or a 200 litre solar geyser means that 1 kilowatt-hours energy has been transferred to the electric geyser saving the same in electricity.

In our situation, the geyser in Sept (Spring) gets to 70 degrees in the afternoon and reduces to 30 degrees after everyone has showered and bathed, meaning it is reducing the electricity power consumption of the house by 10kwh per day. In the high pressure solar geyser, the water in the storage tank is only use to store heat and is not consumed. This means it is suitable for situations where the geyser might encounter temperatures below zero over night as an antifreeze can be added to the geyser water.

The following graph shows the electrical power used by two geysers that use the preheated water from the high pressure solar geyser system. The electric geysers are a 200 litre and a 100 litre system. In South Africa in Johannesburg there is a ripple relay control system that allows the municipality to switch off the geysers remotely during peak loads. This typically happens between 18h30 to 21h00 and from 07h00 to 09h00. The graph shows that for 3 adults 7.26kwh of electrical energy was used to heat the geysers in one day. This situation occured before the load scheduling program was implemented detailed below.

A solar swimming pool filter that runs on a 12 volt pump drawing power from the solar array which is also used by the micro grid-tie inverters. The water is treated with an ozone generator and the main swimming pool pump is only used occasionally to let the pool cleaner retrieve the fallen leaves.

Solar Blankets

One of the most effective energy saving devices is a geyser blanket.

We measure electrical energy daily used to keep our geysers at operating temperature.
We have a 100 litre geyser mounted outside a building - a new design of geyser that was recently replaced and so has the accepted current insulation offered by manufacturers. When running the outer case is not hot - showing the geyser leaks minimal heat. The geyser is on a bathroom that is not used for days and so we were able to measure the energy requirements to replace the heat losses without any consumption.
Before fitting the blanket - the geyser used 1.5kwH per day to maintain heat.
After fitting the blanket - the geyser used 1.0kwH per day to maintain heat.
Geyser blankets are very cheap, easy to fit and very effective.

Load scheduling

Common electronic municipal meters will not give you credit for over generating with the solar/grid-tie system and feeding power back to the neighbourhood. What is needed is to consume the solar energy when it is generated by running non-time critical appliances during the day when power is being generated. We have time switches on our geysers to try to reheat the geysers when the solar power is at a maximum so as to minimise power being returned to the municipality.

Electricity bought from the municipality

Electricity generated and converted by the grid-tie inverters - fluctuation due to clouds

The above graphs show one day operation monitoring electricity bought from the municipality and generated by the solar network. The fluctuations on the solar power is caused by clouds passing between the sun and the panels.

Analysing the upper graph

  1. Base load is shown between 00h00 and 04h00. This reflects power used by security systems, network servers, satelite systems and the fridge and freezer.
  2. Between 04h00 and 04h40 the 200 litre geyser temperature is topped up for morning showers. The electrical geyser power is then shut off with a timer so that it will only be reheated when solar power is at a maximum.
  3. From 07h00 to 16h00 the graph shows how power generated from the solar system starts running the base load reducing the amount needed from the municipality.
  4. Short spikes from 06h00 to 08h30 reflect energy used in the kitchen for breakfast.
  5. From 08h40 to 09h00 reflects power used by the borehole for irrigating the garden.
  6. From 09h00 to 10h00 the solar system is over generating and supplying surplus power to the neighbourhood.
  7. From 10h00 to 11h30 the 200 litre geyser is reheated. During the showering it drew preheated cold water from the 200 litre high pressure geyser reducing the amount of electricity needed for reheating.
  8. From 11h30 to 12h30 the 100 litre geyser is reheated.
  9. From 13h50 to 14h30 reflects power used by the borehole for irrigating the garden.
  10. 19h00 shows power used in the kitchen for making dinner
  11. From 19h00 to 23h00 reflects additional power used by the family for lighting, security lighting and watching television/gaming/reading.

Impact of overcast weather

The above graph shows the total energy generated daily from the solar network. The fluctuations are caused by weather conditions between overcast and sunny days.

The above graph shows the total energy bought on a daily basis from the municipality. On overcast days the energy increases due to the solar geysers not getting as hot as normal and not preheating the geyser inlet water to normal temperatures, meaning extra energy is needed to heat the water to temperature.

We have now introduced time switches on some of the grid tie inverters to match our solar profile to our usage profile.

We have six gridtie inverters in parrallel. Referring to the above graph. At 08h45 one of the units is shut down as the other five are generating sufficient for our base load excluding geysers and swimming pool.
At 09h15 another of the units is shut down as the other four are generating sufficient for our base load excluding geysers and swimming pool.
At 10h00 all units are activated as well as heating of the 200 litre geyser meaning minimal additional power needed from City Council.
At 11h00 the 200 litre geyser is usually at temperature and the 100 litre geyser is switched on.
At 11h30 the 100 litre geyser is usually at temperature and the swimming pool motor is switched on.
The swimming pool motor runs till 15h00 at which time the energy from the solar system has dropped off to the amount needed for the base load.

Delivering 50% of our energy needs from the sun

By analysing the geyser requirements on cloudy days we have worked out the base load needed in the form of hot water for the household. By subtracting the electrical energy needed to maintain this load, we can determine the contribution from the solar hot water geyser systems.

Running our household with its swimming pool, geysers, kitchen appliances, computer systems, borehole, garden sprinklers, entertainment systems, security systems, workshops, compressors, home heating/cooling and all energy requirements except for motor vehicles - typically uses 50 kwh per day of which 50% is supplied from the Municipality and 50% is from the sun.

In the above graph

  • Black trace shows computed total energy requirements on a daily basis - includes purchases from the Municipality, the photovoltaic generated power, and the equivalent electrical power from the solar water geysers.
  • Red trace shows purchases from the Municipality
  • Blue shows the power generated from the photovoltaic system
  • Green shows electrical usage of power to heat the two geysers - we have seperate power meters on the geysers to measure energy usage
  • South Africa is in the Southern Hemisphere and summer is in December and winter in June. This is a 20 room house on a 2000 sq meter stand with a large swimming pool.

    Achieving higher percentages than 50% for the solar is a challenge as we consume energy over 24 hours but only have solar capture over 8 hours to 12 hours. We do not want to go the route of batteries as that defeats the benefits of a grid tie system and increases costs dramatically.

    1 Nov 2020

    Also see an article on battery storage systems
    Also see an article on Energy equivalents