Optimising your Solar Installation
This article was written and submitted by REAG member Pete Walsh
In this and subsequent posts, I will outline what you need to know about acquiring and managing your solar electricity installation. This particular post concerns the use of optimisers in your installation.
Solar Power - a Primer
Firstly solar powered electricity describes the conversion of light (or photons to the Einstein-ly enlightened) to electricity measured in volts (pressure), amps (flow) and watts (power). Simply put, the amount of light hitting your Photo Voltaic (PV) panels directly effects the amount of electric power a.k.a. watts you create to run things. These panels are then connected to an Inverter that converts (inverts) the Direct Current (DC) emitted by the panel array to the Alternating Current (AC) that is used to distribute power on your household, and indeed the external power grid. (More about DC and AC in a later post.)
The problem of Panel Mismatch
When you install an array of PV panels there will be a number of mismatches in the output of the PV panels that will prevent you generating the full rated capacity of the panels. There are considered four reasons for these mismatches:
Manufacturing
PV cells will ‘naturally’ have a power tolerance of up to 2%.
Weather variance
Clouds block sunlight and contrary to normal thinking, heat will diminish the efficiency of a panel.
Module orientation
The position and angle of the panels on a roof will affect the amount of light hitting a panel. Importantly, local shading from (say) chimneys and trees will reduce the amount of available light.
Age
Modules degrade for many reasons, dirt and age being key reasons for loss of efficiency. They will also be variances in the rate in which individual panels degrade.
The special problem of shading
There is little you can do about mismatches caused by Manufacturing or Age. What you commonly can manage is the mismatch resulting from shading caused by the environment e.g. trees, neighbouring buildings. This problem is exacerbated by the use of a single Inverter as opposed to applying an inverter to each panel.
Most domestic solar systems connect the PV arrays in one or (mostly) two strings to a single inverter. Often it is ten or more panels to a string. The mismatch problem occurs by the fact that hooking the panels up in a series effectively ‘chokes’ the efficiency of all the panels. Thus, one or two badly performing panels, what ever reason, reduces the performance of the whole series.
So when some panels are in the shade, even for part of a day (partial shading), then the output of the entire string is compromised.
How Optimising works
Optimisers are essentially boxes that can be attached to a panel. There are basically 2 ways of optimising a series or an array:
1. one way is to not put the panels in a string. This is to do the DC:AC inversion at the panel by attaching multiple micro inverters (one for each panel)
2. the second way is to use an Optimiser which a proprietary technology (software and hardware a.k.a. a ‘box’) to one or more panels connected to the single inverter.
However the issues are different for new projects as opposed to existing installations.
Optimising New Projects
If you are installing a new system, this is by far the best time to consider shading, orientation and other types of mismatches. Following the universal rule of always getting three quotes, here are some questions you should ask your potential supplier:
What is the rated and probable output of the panels proposed? (Note that the rated Tier 1 panels are not necessarily the best)?
Will I have problems with shading AND will it significantly affect performance?
And if so what is proposed to address any potential issues (Micro or optimization)?
Will the proposed solution affect guarantees on any of the system elements (notably the proposed Inverter)?
What sort of monitoring (especially panel level) is included in the quote?
The rest is standard bid management stuff related to cost, quality, trust etc. In Victoria, optimisers are currently used in approx. 2% of installations. Costs will vary but in a typical 5-6 KW system, the Tigo will cost approx. $2,000 for a full i.e. every panel installation. The Solar Edge solution about 10% more. As these boxes are attached at panel installation, this is the most effective time to deploy optimization
Micro Inverters (See option 1 above) effectively isolate each panel from the rest of the array. This means no string inverter is required and no choke points at the panel level. Interestingly, the technology was invented in Australia in the mid 1990s and was actively adopted in the USA. This is an option used mostly for large commercial installations. It is more expensive than using optimisers.
The Optimisation approach offers a more flexible and cost effective solution for domestic systems. It is especially useful for partial shading and for orientation issues where angling of panels is not practical or there is a need for spreading the PV panels across different roofs.
Vendors
There are a number of vendors and technologies in this space. I will summarise three known to be available in Australia.
Solar-Edge is company providing a complete solution to the PV industry. Their proprietary optimization approach provides two or more optimisers (DC:DC) to the solar array at the panel level effectively isolating the low performing panels. Their optimisers however must be connected to their proprietary (DC:AC) inverter. This does however provide a robust and full scale solution.
Tigo is a USA based company with a proprietary solution to optimisation. Essentially it works to isolate choke points in the array through smart sensing and predictive software. It can be flexibly deployed and connects to a range of Inverters. Contacting some installers, Tigo seems to be the preferred solution at this time due to cost and flexibility for existing installations.
Huawei (pronounced “wah – way”) are a major Chinese technology company manufacturing a range of PV related products. Like the Tigo solution, they are inverter neutral and can be attached to solely to the under performing panels. Huawei optimiser ‘prefer’ a Huawei inverter and it would be worth checking the validity of the guarantees.
One other advantage of these extra devices is they can come with remote panel level monitoring functions and rapid shutdown options.
As to the payback or economic viability of using an optimization strategy at your site, well the answer is you may well guess is “it depends”. As there are so many variables, which is why they call them variables, the best approach is firstly, to be informed and secondly always get three quotes.
Australia has the highest uptake of solar globally, with more than 21% of homes with rooftop solar PV. As at 31 May 2020 more than 2.43 million rooftop solar power systems have been installed across Australia.
Take Homes
To consider an optimization strategy; If you are installing a new system:
Determine if you will have a shading problem and ensure it is included in discussions with your vendor(s). Be careful of being oversold.
Most domestic systems will not use micro inverters.
Get panel level monitoring if possible to detect under-performing panels.
Replacing defective panels may serve you well. Note most Inverters do not provide panel-level monitoring so determining a defective panel can be problematic.
You should be able to observe if you have a partial shading problem. Prior to installation is strongly recommended and your installer should be vetted on the potential of this issue.
You may need some expert assistance to determine if you could significantly improve the output of your installation.
Remember ‘money in the bank’ only earns approximately 1%. However spending money on optimisation where there is no measurable gain is never a good idea.
Retrofitting optimisers on installed systems
The problem of mismatches is not more difficult when applied to installed systems, just more expensive.
If you have an existing system and are suffering serious under performance first determine if it is shading problem or some other mismatching problem. Replacing a defective panel may be the solution.
Retrofitting optimisers is an expensive business. Installers recommend installing optimisers to each panel on the affected string. This work requires a re installation of the panels involved as well as re wiring. In addition to the labour, optimisers cost in the region of $150 each. So, (say) 10 panels on an affected string would mean $1,500 plus labour (say $2000) … meaning cost wise you are up in the region of a new system.