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The agony and ecstasy of solar & storage quoting
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.
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.
Apologies for the flickering lights…
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.