Meeting Part L compliance with solar electricity in apartments and large houses

Even for regular houses, the rapid fall in the price of solar photovoltaic (PV) modules has made them worth considering – particularly for larger houses where meeting the renewable energy requirement of 10 kWh/m2/yr requires large buffer tanks and surplus hot water for small families. In such cases it is also possible to meet part L using a combination of solar water heating and solar PV power.
 
Solar photovoltaic systems present many benefits:

• They are entirely “fit-and-forget”. No anti-freeze to replace, no pumps, valves or moving parts.
• When the house or apartment is unoccupied, the system will continue to export power to the grid, or if the property is not connected, the system can simply be left powered off with no ill effect. This is particularly useful for holiday homes, or houses which remain completed but unoccupied for extended periods where overheating would normally degrade antifreeze and put undue wear and tear on a solar thermal system.
• In apartments, cable length is not a significant cost. You can either mount outdoor grid tie inverters on the roof and cable to each apartment’s consumer unit using conventional wiring, or bring the DC cabling from the panels to an inverter in each apartment using double-insulated stranded wire.
• Best of all, the total hardware cost for a 500 W or 750 W system can be as low as €1000 to €1200. Depending on the location and orientation, this would provide enough energy to meet Part L for most apartments and small dwellings between 100 and 150 sq m, as for electricity production the requirement is only 4 kWh per sq m of floor space.

 
Production degradation
Solar PV panels degrade slightly with age. Generally they come with a warranty of 90% output after ten years and 80% output after 25 years. This warranty is usually covered by independent insurance which is a requirement for funding for large solar parks.
 
However, it is normal practice to put a large PV array into a slightly under-sized inverter. This will cap the output of the system for a short period around noon, but will ensure a more stable overall output over the life of the system.
 
Poly –vs- mono
There are two types of module in common use - monocrystalline and polycrystalline. Mono panels will produce very slightly more power per kW than poly. Traditionally this has been matched by a small premium in price, but recent developments in silicone production has widened the gap considerably. At the time of writing, poly is a more economical option, but requires slightly more roof space for the same output. Computer simulations can quite accurately predict the output of a system, given the characteristics of the panel which are available from TUV or similar test results.

Sizing a system
Systems must be sized for Part L using calculations in Deap, based on a fairly simple formula for module capacity in kilowatts (peak), shading and irradiance. Deap assumes a 20% loss. So the formula is:
0.80 x kWp x S x ZPV   (where S = irradiance,  ZPV = overshading)
 
Overshading and irradiance figures used are the same as those used for solar thermal. The output figure of this calculation is entered into the ‘energy produced or saved’ section in the energy requirements tab as renewable electrical energy

System example
Modules – 3 off 240 W polycrystalline modules, feeding a 500 W inverter.
 
In Deap, if the modules are facing south at 35 degrees with no shading, this would show production of 618 kWh, and would meet the part L renewable energy requirement for a house of approximately 154 sq m floor space.
 
However, using more accurate simulation which takes into account the inverter size, this system, should produce 564 kWh per year. After 25 years, the production would only fall to about 520 kWh because the array was initially over-sized for the inverter.
 
If the roof pitch was at a less optimal 45 degrees, and the array was facing south-west, the production after 25 years would fall to 476 kWh – a reduction of just 8.5%.
 
Getting paid for power produced
In this respect, Ireland lags behind the rest of Europe. Until February 2012, surplus power could be sold back to the grid at a feed in tariff of 19c. At time of writing, this has been reduced to 9c. In the UK, a system like this would attract a payment of almost three times that rate, and up to recently the producer was paid a very lucrative £0.42p.
 
So as of right now, the total savings on a system producing 520 kWh would be about €58 per annum, but it is widely expected that this will improve during the lifetime of the system.
 
Getting a smart meter
New houses are provided with smart meters which have the ability to measure exported electricity to facilitate payment by Electric Ireland. This simplifies the procedure, but the connection must first be notified to ESB Networks using a form, NC6, available on the ESB site. The inverter used must have EN50438 approval with some specific Irish standards – standards which the ESB itself doesn’t meet, but that is for another article.
 
The ecological argument
Producing electricity during the daytime is a useful addition to the grid – the energy is produced at a time when industry and many households are consuming power, compared to wind which also produces energy at night during a slump in demand.
 
Electricity is also a premium source of power. To produce 1 kWh of energy in a power station requires 2.6 kW of energy from oil or gas, the remainder of which is usually wasted as surplus heat. This is the reason why the renewable energy contribution from solar thermal is set at 10 kWh/m2/yr compared to 4 kWh/m2/yr for electricity production.
 
In most cases, solar thermal will provide better payback times than solar PV, but for apartments, large dwellings, and households that use less hot water, or are not occupied at all times, the use of photovoltaics will surely become the norm.