LED on Import Electricity Meter Used to Turn on Immersion with Exporting

In our article Flashing LED on Electricity Meter we looked at how the status of the LED on an electricity meter in a grid tied solar PV system can be used to decide when to turn on a water heating immersion heater to use surplus solar generated electricity rather than exporting it.

When there is a dedicated export meter with an LED which flashes at a rate proportional to the power currently being exported, things are relatively simple, but for one client recently we had to deal with a system including only a standard domestic import meter. This has an LED which flashes at a rate proportional to the amount of electricity currently being imported (i.e. purchased from the National Grid), and which is permanently on while electricity is being exported.

Export meter for solar PV system LED detection circuit to power immersion with surplus power

The client wanted a device which would turn on his immersion (standard 3kW element, but powered via a power reducer which halves the power consumption to 1.5kW), after a user programmable number of minutes of continuous electricity export. The exact number of minutes desired for efficient operation was unknown, so we made the device programmable – i.e. the user could themselves set the number of minutes of continuous export required before the immersion would be turned on.

The immersion would then remain on until 5 seconds of the import meter LED flashing. Therefore, if turning on the immersion results in electricity being imported, the immersion would be turned off within 5 seconds, so very little electricity would be imported to power it. While the amount of electricity taken by the immersion is insufficient to use up the full export surplus, the immersion would stay on, heating water.

Surplus solar PV immersion controller using export meter LED to decide when to turn on immersion

Obviously this is not the most efficient system possible – something with a current sensing clamp to detect the exact level of import or export power is better, but the commercial options with this feature start at around £130.

This simple and easy to install system ensures that on a sunny day when no-one is at home using a high powered appliance such as a kettle or washing machine, surplus electricity from the solar panels will always be used to heat water rather than being exported (for which just a few pence would be paid), resulting in a payback period measurable in months.

The status of the LED on the meter (on or off) is detected using a simple light detecting resistor (LDR).

If you would like something like this device, please email neil@reuk.co.uk with details of your requirements.

Dawn Dusk Lighting Controller with Regulated Output

Pictured below is a modification of our standard Mini REUK Dawn/Dusk Lighting Controller which has the addition of a regulator on the output so that it can be connected directly to LED lighting, and connections to enable a PV solar panel to be connected to the battery via the board.

REUK Dawn Dusk Lighting Controller with Regulated 12V OutputWe used an LM2940CT-12 regulator on the output side which will supply up to 10 Watts of 12.0V output to protect sensitive LED lighting from excessive voltage from the solar charged battery. (If the battery voltage falls below around 12.3V, the output voltage will fall to around 0.3-0.5V below the battery voltage).

We added a Schottky Diode to solar panel input so that the battery will not drain charge through the solar panel at night. A Schottky type diode was chosen as only around 0.3V of solar voltage is dropped through it compared to the 0.6V+ typically dropped through a standard silicon type diode.

Automatic Low Voltage UPS Shutdown

Pictured below is a special low voltage disconnect controller designed to switch off an uninterruptible power supply (UPS) when the battery bank to which it is connected is low on charge.

Low voltage detecting automatic shut down for Uninterruptible Power SupplyThe system for which this controller was built is as follows. A 72V 2.3 kW solar PV array is connected to a 48V 50A MPPT solar charge controller. The charge controller charges a 48V battery bank (made up of an array of 12V batteries); and that feeds a 3.5 kW UPS  which supplies electricity to the house.

The UPS does not have a low voltage disconnect feature and so from time to time the batteries were being discharged too far. Obviously the owner wanted to avoid this as the battery bank is a very expensive part of the system.

This particular UPS has a standard female USB port on it, and if its pin1 is shorted out with one of its other pins, the UPS switches off.

Therefore our controller is a slightly modified version of our standard programmable low voltage disconnect. One of the 12V batteries in the battery bank is constantly monitored. If the voltage measured stays below a user set low voltage threshold (e.g. 12.0V) for 10 seconds, the on board relay closes for 2 seconds which shorts two of pins in the USB socket and forces the UPS to switch off immediately. Therefore the battery bank is protected from being run down excessively.

Mains Backup for Solar Charged Battery Bank

Pictured below is a controller designed to automate backup mains charging of a battery bank which is charged by an array of photovoltaic solar panels.

Main charger back up for solar pv charged battery bankThis controller is based around our low voltage disconnect products. It constantly monitors the voltage of the battery bank. When the voltage falls below a user set low voltage (e.g. 12.0V), the on board relay closes which connects a mains powered battery charger to the battery bank.

When the battery voltage then gets up to a user set high voltage (e.g. 13.6V), the on board relay opens which disconnects the battery charger.

This controller is fitted with manual overrides so that the user can ‘override’ the controller and force the mains powered battery charger to come on when off, or turn off when on.

This is very useful for a solar powered system since the battery bank voltage may be getting low and the weather forecast may also be bad for the coming day. In that scenario, it is worth charging up the battery bank fully immediately to prevent the battery bank from hanging around at a low voltage for an extended period of time (which is bad for battery longevity).

Similarly, if the mains charger is on because the battery bank was low on charge, but the weather forecast is for non-stop sunshine, the override enables the charger to be turned off to leave the solar panels to do the rest of the work.

If you need a controller like this, email neil@reuk.co.uk with details of your exact requirements.

 

Playhouse Lighting Controller with LVD

Pictured below is the connection diagram for a controller we have built to be used in a child’s playhouse. The playhouse will have a solar charged battery which will be used to supply power to three LED spotlights with a light switch, a small fan to circulate air and prevent the playhouse getting damp or too hot, and a dual cigar lighter socket with USB sockets which will be used to charge battery powered gadgets.

Connections for low voltage disconnect with twin outputs - regulated for LED lighting and to power a fan for ventilationAs the playhouse will be used by children, safeguards have been fitted to prevent the battery from becoming excessively depleted and permanently damaged.

In order to prevent the lights being left on and forgotten, when the light switch is turned on, a timer starts and if after one hour the light switch has not been turned off, the lights turn off automatically and will not turn back on until the light switch is toggled.

The output from the controller to the LED lighting is regulated to 12.0V so that excessive voltage (particularly when the batteries are being charged) does not damage the voltage sensitive bulbs.

To reduce power consumption, the fan which is used to ventilate the playhouse is turned on for just one hour every six hours automatically by the controller. The output to the fan is not regulated as the fan is rated for use with up to 18V, and if the battery voltage is high it is because there is or has been a lot of sunshine. Therefore the faster fan speed will prevent the playhouse getting too hot and stuffy.

In order to prevent the battery from running too low on charge, an automatic low voltage disconnect is incorporated. When the battery voltage falls below 11.9V, the outputs to the fan and lighting turn off, and only turn back on again when the measured battery voltage exceeds 12.4V.

The cigar lighter / USB socket is connected directly to the battery as the gadgets being charged by it will not draw much current for long if left connected since their batteries will get full and charging will stop automatically.

If you need a controller such as this, or with any of the features it has, email neil@reuk.co.uk with details of your exact requirements.

Project of the Day – Immersion Controller using Electricity Meter LED

Our standard Surplus PV Immersion Controller uses a light detector to estimate when the solar electricity generation from a domestic PV array is above a user-set threshold. It then turns on an immersion heating element to use the surplus electricity rather than exporting it to the grid.

REUK

This simple light detector approach works particularly well in homes where the householder is out all day since once the baseline electricity needs of the home are met, the surplus electricity can be used for free water heating instead of being exported (which would earn just a few pennies per kWh unit of electricity exported).

However, this system is not a true surplus electricity controller since if the home owner has a 4 kW solar array, and a 3kW immersion element, then if more than 1kW of electricity is being used in the home on other things – e.g. kettle, washing machine, or other heavy loads, electricity will be imported at a cost to cover any shortfall.

This week we have been working on the next level approach which is very useful for some households depending on how their electricity meter(s) are set up. In our article Flashing LED on Electricity Meter we looked at how modern electricity meters are fitted with a red LED which flashes at a rate proportional to the power passing through them. They are marked with something like 1000 Imp/Kwh which means that the LED will flash 1000 times per kWh unit of electricity passing through.

The meter for which we made this modified controller this week has such an LED which flashes while electricity is being exported and is off while importing. The flash rate is 1000 Imp/kWh and so if 1000 Watts of electricity is being exported, the LED will flash once every 3.6 seconds; if 2000 Watts of electricity is being exported, the LED will flash once every 1.8 seconds; if 100 Watts of electricity is being exported, the LED will flash once every 36 seconds, etc.

Using this information we modified the programming on our standard immersion controller so that instead of the light detector measuring the level of solar radiation and therefore estimating the power generated by the solar panel, it now measures the LED flash rate on the electricity meter so that it knows how much electricity is actually being exported net of any loads in the home.

half wave rectification of electricity to halve power use of immersion element

This controller is being used with a 1.5kW immersion heating element which is powered via a diode to give half-wave AC electricity cutting the power of the immersion element in half to 750 Watts. The solar array is rated at 4 kW.

We programmed this particular controller to turn on the immersion element when 1,100 Watts or more is being exported and then turn off the element when 200 Watts or less are being exported. If 1,100 Watts are being exported then when the immersion heating element turns on, the export will instantly fall down to 350 Watts, and there is an extra 150 Watts of hysteresis in place so that the immersion will not be turned off when the power taken by the devices using electricity in the home fluctuates a bit.

If you are interested in this type of modified immersion controller, please email neil@reuk.co.uk with details of your electricity meter(s), immersion power rating, and solar array power rating.

Surplus PV Solar Immersion Controller for Solid State Relay

Pictured below is a modified version of our standard Surplus Solar PV Immersion Controller a device designed to use electricity generated by photovoltaic solar panels to power a water heating immersion element rather than exporting the electricity to the grid.

REUK Immersion controller for Solid State Relay (SSR)

This version works identically to the standard unit, the only difference is that instead of having a mains switching relay on the circuit board itself, there is a 12VDC output when then solar generation is above the user-set threshold which can be used to control a solid state relay or other relay in situations where the 16A relay we usually supply would not be up to the job – e.g. for a very large immersion element.

25A solid state relayPictured above for example is a 25A rated SSR (solid state relay). The 12VDC output from our controller connects to the terminals labelled 3 (+) and 4 (-), and then the mains live cable is switched through terminals 1 and 2.

Click to buy Solid State Relays – prices start from around £3 delivered in the UK, and send an email to neil@reuk.co.uk if you are interested in this modified version of the immersion controller.

Special Offer of the Day – 20W PV Solar Panel £25.99

While the price per Watt of PV solar panels has been tumbling, the cost of smaller solar panels (<30 Watt) have remained relatively high in most cases because of the fixed costs of postage, and manufacturing without the benefit of scale.

Smaller panels are particularly useful for battery powered outdoor projects such as shed lighting, electric fencing, irrigation systems, and much more.

20 watt PV solar panel special offerPictured above is our special offer recommendation of the day – a high quality 20 Watt monocrystalline PV solar panel for just £25.99 plus £2.99 postage (and no extra postage if you purchase 2 or more of these panels).

This panel is 363mm x 529mm x 25mm in size and 2.5Kg in weight, and is fitted with 1.5 metres of cable and a bypass diode to prevent reverse discharge at night.

Click here for more information or to snap up one of these panels now: 20 Watt PV Solar Panel Offer.

Reducing Reflectivity of PV Solar Panels to Increase Efficiency

The US Department of Energy’s National Renewable Energy Laboratory (NREL) is carrying out research on reducing the reflectivity of PV solar panels to increase their efficiency.

Standard silicon solar cells (out of which solar panels are built) reflect around 35% of all the light which hits them – therefore one third of the solar energy hitting them is immediately lost. To reduce these losses, commercial solar cells receive an anti-reflective coating (gas molecule deposition) which brings down reflectivity to 7-10% typically. (It is this coating which makes solar cells appear blue). This process is toxic and the equipment used to achieve it is very expensive. Therefore NREL are looking at ways to reduces reflectivity of solar cells while at the same time reducing costs of manufacture.

NREL Natcore technology black silicon solar cellNREL’s solution called black silicon uses an acid bath with a silver nitrate catalyst to put trillions of tiny holes into the surface of each 6×6 inch solar cell. These holes trap the photons of sunlight increasing the chance they will be absorbed by the solar cell and converted into electricity. With this technique reflectivity is reduced to less than 1.5% (which results in solar cells which appear black).

An added benefit is that solar cells treated in this way perform better in the morning and afternoon when the angle of the sun is further from the optimum (perpendicular), and they also perform well in overcast conditions.

NREL black silicon under microscopeNREL have exclusively licensed this black silicon technology to Natcore Technology  specialists in thin film solar panel manufacturing. With further development they hope to make more efficient and cheaper solar panels than anything on the market today.

Saudi Solar Power Boom About to Begin

Reuters yesterday published a very interesting analysis article on the future of solar power in Saudi Arabia. Perfectly located for solar power generation, Saudi Arabia currently has just 12MW of installed solar capacity – less than was installed in the UK in the first half of May, and a long long way behind the 5,000MW of capacity installed in China during 2012.

solar power boom in saudi arabia

Saudi Arabia is has relatively low natural gas reserves, but vast oil reserves, so Saudi power stations are oil fired power plants which give an efficiency of just 30%. Since oil used in domestic power generation is oil which cannot then be sold on the international market, there are billions of dollars per month of opportunity cost, effectively pricing Saudi electricity generation at a huge 26 cents/KWh.

With the price of solar panels falling to US$0.80 per Watt, and assuming installed costs of US$1.50 per Watt for utility-sized solar power plants, solar generation costs would be just 9.6 cents/KWh – less than half the cost of using domestic oil.

For these simple economic reasons, the Saudis plan to install 5,000MW of solar over the next 5 years, and 41,000MW over the next 20 years which will give them billions of additional barrels of oil to sell on the international market.

Click here to read the full Reuters article.