Category Archives: Water heating

Solar PV Installation – 1 year on

solar-panel-ikea

We’ve now had our solar panels for 1 year, so it would seem time for an update on how things are going. Our 4kW SSE-facing system has an expected annual generation of 3,668 kWh. However after a year in service I’m pleased to see that we’ve generated 4,056 kWh – 10% more than expected.

The financial returns on a solar PV system are a combination of 2 things: (i) payments from your chosen electricity company for energy generated and exported to the grid and (ii) savings from not having to buy so much power as you use that which you’ve generated instead.

When it comes to payments from the electricity company, my electricity company (like many) chooses not to go to the expense of installing an export meter and instead assumes that half of the power which I generate is exported to the grid. The total annual revenue for the first 12 months of operation, including both generation and export (assumed to be 50% of generation) is £629.62.

Then there’s the question of how much electricity I save. I’ve only had monitoring of usage since March (approximately 6 months) but in that time I’ve used 41% of the generated electricity to replace bought electricity. 41% of 4,056 kWh is 1,663 kWh. What is less clear, is what the unit saving for this energy is. Some of this is daytime usage like standby loads, the fridge, cooker and other daytime loads at 11.7 p/kWh; but some would otherwise be night time loads like dishwasher, washing machine, or car charging at 7.57 p/kWh. I don’t measure the split so I’m simply going to assume an average unit rate = (11.7 + 7.57)/2 p/kWh = 9.6 p/kWh. 1,663 kWh @ 9.6 p/kWh = £159.65.

Finally, there’s the question of how much gas I save by making hot water from solar PV electricity rather than gas. Since March I’ve used 813 kWh or 27% of the generated power for water heating, so for a whole year 27% of 4,056 kWh is 1,095 kWh. The immerSUN itself records 995 kWh used for water heating since December. I’m also going to assume that not all the heat from the gas boiler would have ended up in the tank as hot water since some is lost via the boiler flue to the outside world, and some is lost via the pipes to the inside of the house – so let’s say 80% efficient on gas. 1,095 kWh @ 3.01 p/kWh @ 80% efficient = £41.20.

The combined total of my revenue and savings for a whole year would have been £830.46 – a 13.6% return on the investment or payback in 7.3 years.

The tariff scheme will have provided me with about 20 years of income by the time it closes, so the investment in the panels, as well as helping me reduce my carbon footprint, will have generated 12+ years of profit having paid for itself during the 8th year.

Making best use of energy

If I want to make best use of my solar PV (i.e. use as much of it as possible) then it seems logical to prioritise replacing my most expensive energy purchases. Examination of my bills suggests that my different sources of energy have the following costs:

  1. Day-time electricity: 12.31 p/kWh
  2. Night-time electricity: 8.02 p/kWh
  3. Any-time gas: 2.87 p/kWh
  4. Solar PV (when available): 0.00 p/kWh

In winter when little PV electric is available our normal practice is to shift as much electric load as possible to night time use saving 4.29 p/kWh by use of timers. Washing machine, dishwasher, and electric car charging all get shifted to Economy 7 by use of timers. The washing machine is the oldest and uses a external timer plug while both dishwasher and car have their own inbuilt timers allowing consumption to be delayed.

In summer the same approach can be used to shift these loads to daytime to make use of solar PV although if the weather is dull and cloudy there’s no guarantee that sufficient PV will be available risking consumption of bought daytime electricity.

My Immersun already minimises exported/surplus PV by running the immersion heater to mop up surplus electricity, although as gas is cheap the savings are smaller than might be achieved by avoiding a similar amount of night-time electricity consumption. In winter we minimise gas use for water heating, while still providing a gas safety net by displacing in both time and temperature. The Immersun naturally heats water in daylight hours only and generates hot water at 60C (not adjustable) as defined by the immersion heater thermostat. The gas is set to complement this being set by timer not to overlap Immersun hours and is set to 50C by the cylinder thermostat. The water temperature difference is not noticeable in practice especially as the showers are thermostatically regulated.

Car charging 1The device that consumes most electric power is undoubtedly my electric car. Its usable battery capacity is about 10.5 kWh and, with charging efficiencies, it can take up to 12 kW of mains electricity to charge (less in summer). The appeal of charging this on solar PV is high, although the cost of doing this if it inadvertently draws day-time electricity is also high (but still much cheaper than petrol or Diesel). It occurred to me that it would be good to use spare channels on the Immersun to control car charging as my largest consumer of my most expensive fuel. It has three channels – two variable power and one on/off – although it can be configured to produce three variable outputs using the on/off to switch between devices.

It seems to me that there are three alternatives for control:

  1. Proportional control.
  2. Stepped control.
  3. On/Off control.

Proportional control would be the most capable. The Immersun has the capability to provide proportional control to multiple resistive loads, and the Mode 3 communications standard to which virtually all electric cars adhere also provides for the charging equipment sending a proportional signal to the car via PWM, so there’s scope for a proportional system. However it seemed that the time to develop this was beyond what I have available, and in practice although the car would receive a proportional control signal it would normally respond in a stepped manner so there’s little to be gained by investing my time to develop a fully proportionate system..

Stepped control is easier to achieve. As it happens my car model is more steppy than some as I believe that it switches between 6, 10 or 15 Amps consumption only. 15 Amps is very close to the full 4 kW output of my panels and, given that there are always other loads in the house, then I don’t think it would ever use this setting, which leaves me with switching between 0, 6 and 10 Amps. Unfortunately of course you can’t switch between 3 states with an on/off signal that only has 2 states, so I’d still need to decode an analogue Immersun output or invest in a second Immersun to get another on/off output but I deemed the latter cost-prohibitive. Generating a variable current signal to the vehicle would also require some logic to reduce the pulse width on the PWM signal on demand – not impossible but still requiring some time to create.

That leaves me with on/off control at a fixed current. This is readily achieved from the Immersun which has a relay output that can be configured to operate when a certain solar power is available (such as 1.5 kW to drive a 6 Amp charger). I decided not simply to turn the whole charger on or off by switching the power as this would cause the mains contactors in the charger and the HV DC contactors in the vehicle to drop out under load which is bad for longevity. Instead I’ll interrupt a low voltage control signal within the charger to trigger stop and restart in a more controlled manner – effectively car and charging equipment will see the signal as the user wishing to disconnect the cable as you might if you wished to drive before charging was complete.

Finally I considered how to connect the Immersun to the charging equipment. Unfortunately the Immersun is in the airing cupboard upstairs (where it drivers the immersion heater) and the garage where the car charges is a separate building alongside the opposite side of house. Neither a second Immersun or a physical cable looked like an attractive prospect on cost grounds so I decided on radio control using around £20 of home automation parts and a total investment of around £40. More on this solution over the next few weeks.

Solar panels can generate significant amounts of electricity for example on March 25th our 4kW panels generated 22.4 kWh over the course of the day as shown.
ImmerSUN 25-03-2016
This generation typically substantially exceeds the load being drawn by the house leading to export of electricity to the grid – 12.3 kWh in this example or more than half the generated energy.

Moving loads such as the washing machine or dishwasher from night to day can help use this electricity, but done to excess risks increasing day time electricity import if changing cloud cover or other factors reduces generated power. The varying power demand from such devices is also unlikely to coincide the the surplus power available leaving some unused surplus. I was thus pleased to come across the ImmerSUN system.
ImmerSUN
The ImmerSUN is a device which diverts surplus power to a range of possible consumers such as an immersion heater. It has a current sensor that measures any surplus power being exported and then diverts a similar amount of power into the immersion heater. Normally of course an immersion heater is either on or off, but the ImmerSUN provides proportionate control between 0 and 100%. It can control several devices diverting power to a lower priority device once, for example, a higher priority immersion heater has raised the water temperature to the set point and power for that purpose is no longer required. In the top illustration the blue bars show 4.7kWh being diverted into water heating (and thus reducing my gas bill) that otherwise would have been lost to export.

For further details see ImmerSUN website .

At the time of writing if you order online quoting Referral Code 206505 you will receive £25 in department store discount vouchers or a free system upgrade to remote monitoring.