Edwin Cotter (engineer) from SolaX chats to Glen Morris about their new range of inverters launched at All-Energy 2017.
Of particular interest was their new, long awaited, IP67 single and three phase hybrid inverters. These can be installed in master/slave configuration and “clustered” with up to ten units to build a 100kW power station with embedded management from the master unit.
The battery storage revolution has long been the subject of hype and speculation but homeowners have now taken to embrace the smart energy model in droves.
Battery storage technology requires a much higher level of sophistication than grid-connect solar to model energy flows and provide an accurate picture of the energy, component lifecycles and, thereafter, financial benefits to the owner.
To provide a recommendation encompassing grid consumption, solar & storage, and supplying the consumer well-informed options, requires a greater level of modelling power.
The spreadsheet is history
For years many installation companies have prided themselves on their tailored spreadsheets to size solar systems.
These may have served to produce a rough estimate for small solar panel installations but can a spreadsheet really model the factors below?
Firstly, the most variable factor in home energy systems is the habits of the energy users themselves.
How can you assess the optimum size, components and configuration of a solar and storage system without load profile data and the year round variability?
Ideally, actual ‘interval data’ of this consumption will be available either from an electricity provider or via a monitoring device installed to gather this data.
Next the tariff plan should be available to measure the financial benefit including the variability from hour to hour and weekday to weekend.
For a thorough analysis, a variety of time-of-use plans available to the consumer from competing retailers is required to compare for the best option.
With this information in hand we can begin to run a generation and energy flow model.
What’s so difficult about that?
A proper modeling algorithm will evaluate solar production for each hour in each day of the year, using a ‘typical’ year profile, which builds in hour-to-hour variability based on cloudiness indexes and using satellite derived solar radiation statistics within a close proximity.
While the solar radiation is in hourly intervals, if consumption data is in 15 or 30 minute intervals, this is the resolution that our modeling should occur to best model battery charge and discharge cycles.
Within each interval, the algorithm should evaluate beam, diffuse and ground reflected irradiance impinging on the panel, while factoring any shading and transmittance losses.
Solar panel output should be deriving using hourly temperature and other derating factors, then power losses through cabling, peak power clipping and efficiency losses at the inverter before arriving at the usable power.
Then self-consumption of solar can be calculated and battery charge and/or solar export allocated, with various user or configuration limits applied to each.
Batteries – quantifying a dynamic system
One of the reasons for the success story that is solar panel systems are the most predictable of generation and component lifecycles.
While the technologies vary greatly, batteries generally share the dynamic nature of the electro-chemical conversion process.
The measurement of the state-of-charge of a battery at any given time is an estimation based on known factors and in some cases unknown factors.
The variability of which is primarily affected by:
the depth of cycling as set in the inverter configuration, varying by daily generation and consumption changes,
charge current levels with consideration of battery loss factors,
discharge current levels, with effect varying by battery technology according to Peukerts law, and with current varying as different appliances are turned on and off, and
the temperature of the battery.
Therefore a model that repeats this energy analysis in smaller time intervals provides a better evaluation of battery cycling.
Analysing these energy flows in each time interval throughout a year and applying the various tariffs allows us to estimate not only the self-consumption, but the storage cycling, and the financial advantages of one system over another.
But it’s not that simple…
Tariffs aren’t tariffs
We are blessed in Australia with a history of electricity monopolies and policy that has resulted in consumers facing huge variations and complexities in the tariff structures.
We’ll leave the greatly increased complexity of commercial tariffs and gross vs net feed-in tariffs for another time.
To summarise, most consumers have a single rate tariff, or time-of-use tariffs in which higher rates are applicable during afternoon/evening peak periods, or some will have a block tariff with variable rates based on the overall level of usage over a given period.
In addition, daily supply charges and discounts may applied, along with variations in feed-in tariffs paid for solar power export.
But wait, there’s more
Having arrived at production, cycling and financial values for a typical year is well and good but a value proposition must also include the cash flow variations over the lifetime of the system and the lifecycle of each component and costs of replacement within the period of this return on investment.
Again batteries add much complexity due to their dynamic nature and technology variability in the estimation of their lifecycle. There is little consistency in warranty conditions so each battery product must be evaluated on its own terms to arrive at the use by date for modelling purposes.
Finally it gets simpler
A few larger companies may have the resources to model of all these factors with a good degree of reliability and provide consumers with the level of transparency that they deserve in an investment such as this.
Consumers have begun to get educated in the factors in their buying choice and are demanding more of sales and install companies, as they should.
Nobody will benefit from budget solar battery operators taking a hold in today’s market.
At Solaris, we have developed SolarPlus V3 over six years to perform these modelling tasks and much more to allow solar and storage salespeople to get from zero to hero in just a few minutes.
There are always challenges in responding to each new technology coming onto the market but that’s the pain we take on for our users to experience the ecstasy.
Glen Morris unpacks the latest cool solar and battery storage products at the Smart Energy Lab. SolarEdge have supplied their new ground breaking HD Wave inverter (light, powerful and easy to install) and Alpha ESS have supplied their new IP65 hybrid inverter/battery system called the SMILE5. It’s a 5kW hybrid inverter with dedicated backup circuit and multiples of 5.2kWh battery packs that can be stacked vertically or horizontally. Pretty cool looking unit.
So called “a.c. coupling” is one of the easiest ways to add a battery storage system, with or without additional solar panels to an existing solar installation. In Figure 1 above, the “Battery Inverter” has been added to “couple” the stored energy in a battery to the switchboard (Load Centre) of the installation. The key component though is the “Energy Meter” upstream of the loads and existing solar generation. This meter allows the battery inverter to “see” the flow of energy into or out of the installation and choose whether to export battery energy to match the energy being consumed by the loads (but not meet fully by the existing solar system).
Put simply, the loads are supplied first by the existing solar inverter and any extra is then supplied by the battery inverter. If both of these is insufficient the the difference is sourced from the grid.
This all happens seamlessly, thanks to the magic of Kirchoff’s circuit laws and to the information that the energy meter supplies to the battery inverter.
DC coupling of batteries to solar PV system
If installing a new solar and battery storage system then d.c. coupling is one of the most popular options as the equipment manages both the battery and the solar generation within the one unit. So called “hybrid” inverters are those that can have both solar PV connected and battery storage.
The advantage of d.c. coupling is that the inverter is only used to converter d.c. to a.c. to supply the loads at the installation – internally the solar is directly charging the battery via a d.c. to d.c. path at very high conversion efficiency.
In Australia and New Zealand where the grid-connection standard AS/NZS 4777.1 applies – total Inverter Energy System (IES) capacity for a single phase installation must be <5kVA and thus limits the number of inverters connected to a total of 5kVA (approx. 5kW). This can be augmented by the local electricity network supply authority (typically with export limiting) but does make adding more a.c. coupled battery storage somewhat limited by total IES capacity of the site.
In Figure 3 above the d.c. coupled hybrid system has no backup circuit. This is not an uncommon arrangement and best suits those customers who merely want to shift energy between solar, storage and their loads.
Backup functionality adds cost and complexity and is not aways available with all hybrid battery storage products.
When sizing a battery storage system for a hybrid solar system it is important to consider to objective. If supplying all the energy to the installation by a combination of solar PV and stored battery energy then the customer’s load profile needs to be carefully considered.
In Figure 4 above you will see that the battery’s State of Charge (SOC) reaches 100% just after midday. This would indicate that the battery capacity is too small to avoid “spilling” solar energy out to the grid and thus loosing the potential savings it might offer.
Also, the energy supplied from the battery to the loads in the evening is capped at 3kW due to the inverter’s limited battery power and thus considerable grid sourced energy is being drawn in during the peak early evening period to make up the difference.
So both battery capacity and inverter power ratings need to be matched to the customer’s load profile.
The “Dream Team” is our test configuration for adding extra storage capacity using lithium ion battery storage to an existing lead-acid off-grid system.
The configuration consists of a Schneider XW+ with both Conext 600/80 MPPT and 3kW RL charging a 600Ah Neuton Power VRLA (sealed) lead-acid battery bank. This system is a.c. coupled to a SolarEdge SE5000 with Backup Unit attached to the new LG Chem RESU9.8H (400V) battery.
The key to making this work is the SolarEdge supplied Wattnode energy meter installed in the main switchboard. The configuration of the SE5000 is to maximise self-consumption and thus the meter tries to preference the use of SE5000 connected solar (4kW with SolarEdge Optimisers) and the 9.3kWh (usable storage) of the LG Chem battery.
When either power or energy is insufficient to meet the load requirements then the XW+ supports the loads.
Note: SP Pro has been substituted for XW+ in this installation.
The main benefit of this “Dream Team” configuration is that it allows adding additional storage capacity to an existing lead-acid system. The new lithium battery does most of the daily cycling and thus extending the life of the lead-acid battery.
AS/NZS 5033:2014 has an exemption to the requirement to have a switch-disconnector between the PV array and the PCE (inverter, charge controller or load) for DC Conditioning Units.
“The input circuits between the PV modules and the d.c. conditioning units are not required to have load break switch-disconnectors provided the input to the d.c. conditioning units is arranged so that the following applies: … (c) Each input is limited to 350 W maximum PV power at STC and a maximum input voltage no greater than ELV.”
However, back in 2014 maximum module size was around 250W and this 350W limit seemed to allow any single module to be connected to a DCU such as those made by SolarEdge, Tigo etc… now, just three years later modules are heading over 300W and some nudging the 350W limit already.Interestingly, module manufacturers such as those listed below, already required covering of the module with an opaque material to limit any current before connecting or disconnecting the plug and sockets of a PV module. This manufacturer’s requirement provides a safe way to attach modules >350W @STC to a DCU and still ensure that the power in the cables is kept below 350W.
Covering with an opaque material also allows a safe (and no plug/socket damaging method) to disconnect a PV module from a faulty DCU (has gone short circuit internally).
I would argue that since the intent of the standard is to limit the power supplying the DCU to <350W, if following manufacturer’s installation instructions and covering the modules before disconnecting plugs and sockets, then modules larger than 350W@STC can be connected to a DCU and still meet the “no switch-disconnector on PV to PCE” exemption of clause 2.1.5.
List of module manufacturer’s who’s installation instructions require covering of the module with an opaque material:
Some of the exciting new systems at the Smart Energy Lab
Raj showing Mark how to commission the NicestESS 5kW AC coupled battery inverter. This unit can be used to add energy storage to any existing PV inverter system. The NicestESS unit is wired between the existing PV inverter and the switchboard to which it connects. The unit takes 48V Pylontech US2000B batteries.
Shaun and Steve discuss the Victron Energy system being installed into the new IP54 enclosure from Power Plus Solutions. The inverter is the new AS/NZS 4777.2:2015 compliant Multigrid 3kW hybrid inverter. The battery system is a GenZ pack of up to 6 x 3kWh units in the cabinet below.