Solar PV + storage
Solar PV + storage
Table of Contents
General
In most places, there is a peak in electricity demand in the late afternoon and early evening. Solar PV panels clearly cannot generate beyond dusk, which leaves a big gap in the early evening. However, 4 hours of storage allows daytime solar PV output to be spread to cover the evening peak.
There are options for the connection between the solar and the storage as follows:-
- Independent – the battery storage system is completely independent of the solar PV farm and does not share a grid connection. There are no constraints on the charging or discharging of the storage.
- AC coupled – the battery storage system and the solar PV farm share an AC grid connection with limited maximum capacity (both outwards and inwards). But otherwise, the two can operate independently.
- Loosely DC coupled – the two share a grid connection and a bidirectional DC to AC inverter. The storage can be charged from the solar PV output via a DC to DC connection, and from the the grid via the inverter subsystem.
- Tightly DC coupled – the two share a grid connection and a DC to AC inverter. The storage can only be charged from the solar PV output, via a DC to DC converter.
Although storage is expensive right now, compared with solar PV technology, a few things work in its favour, though detailed costs matter a great deal.
Firstly, for utility solar PV, DC capacity is normally configured considerably higher than the AC grid connection capacity – a ratio called the ILR (inverter loading ratio). Because the sun isn’t always shining directly in daytime, the optimum financial return from a project for US fixed tilt C-Si systems in 2021 used an average ILR of 1.38.
The implication is that, around noon on a sunny day at mid summer, nearly 40% of the possible solar power will be clipped by the inverters, and go to waste if there is no storage. Adding storage to a financially optimised solar PV project captures this energy, otherwise wasted, to charge the battery, then sold at peak evening electricity prices. The more sunlight in a location, the lower the cost of the stored electricity.
For ILRs of 1.3 or less, only a few percent of DC power from the solar panels will be wasted due to clipping.
Secondly, solar panels are less than half of the total utility solar PV farm capital cost. In particular, DC to AC inverters and a grid connection are quite expensive, and some configurations solar plus storage enables these and the AC grid connection to be shared between the solar and the storage.
Note this is only an advantage for storage connected by a DC link to the solar panel output. If a solar plus storage configurations is connected by an AC link, then the installation carries the cost of independent, high quality, DC to AC inverters for the battery storage. However, the grid connection can still be shared.
Thirdly, in regions where a lot of solar PV capacity is installed, such as California, wholesale power prices are generally higher in the evening than during the daytime. Even if power from storage is more expensive than power direct from solar PV, it may well be worth selling it if prices after dusk are a multiple of noon prices.
And lastly, some auctions are open only to solar PV systems of a maximum AC MW capacity. Provision of storage enables more power to be sold for the same connection MW capacity.
The financial optimisation of a DC coupled solar PV + storage system, including the ILR ratio, will be different from that of a standalone solar PV system.
Cost examples
- $21.00/MWh 100 MW solar plus 50 GW of storage, Buena Vista, Otero County – New Mexico grid May 2020
This cost is low compared to the typical case which shows the LCOE of solar PV plus storage farm as at least 70% more than the LCOE of solar PV farms without storage.
The NREL report : US Solar Photovoltaic System and Energy Storage Cost Benchmarks, Q1 2022 [p57] gives the capital cost of a 100 MWAC solar PV farm without storage as $117-133m. Add 4 hours of 60 MW of storage to this (total 240 MWh), and the capital cost rises to $200-229m.
Selected country information
As of April 2022, in a report by renewable UK, 6.1 GW, or nearly 20% of the UK grid battery storage backlog of 32 GW was planned to be co-located with a solar PV farm, mostly in England. The grid battery storage backlog doubled from the report 12 month previously.
The high value for storage combined with for UK solar may be because the terms of the UK 2021/22 CfD (contract for differences) AR4 (allocation round 4) restricted the maximum solar farm output to 50 MW.
One way to sell more power under this restriction is to provide considerably more then 50 MW of solar panels, and thus a high ILR (inverter loading ratio), and store the excess power for later discharge to the grid. By increasing the AC capacity factor in this way, clearly the total revenue is increased. Optimising the profit depends on the cost of the individual components (solar panels and mounting, inverters and storage.
2.2 GW [ table (E)] of solar PV was contracted for installation in UK financial years 2023/24 and 2024/25. The government has said the CfD auctions will be annual from now on. The pipeline of 6.1 GW of storage capacity co-located with solar farms may be going through the prerequisite planning processes to support solar plus storage bids in CfD auctions over the next few years.
According to NREL [slide 28], from 2017 to 2021, 2.4 GW or 7% of US utility solar PV farms included storage. For these solar farms with storage, the storage averaged 80% of the MW capacity of the solar PV output (total 1.9 GW), and the 6 GWh total represented an average of 3 hours duration storage.
However, NREL says the EIA expects between 25% and 32% of new, utility solar farms installed from 2022 to 2024 to include storage.
From the alternative perspective, the US EIA expected that two thirds of the 14.5 GW of the US battery storage capacity likely to commission between 2021 and 2024, will be installed alongside a solar PV farm.
A document from NREL “US Solar Photovoltaic System and
Energy Storage Cost Benchmarks : Q1 2022″ shows the capital costs of solar plus storage per kW of solar capacity to be around twice as much as just solar (with no storage). In this case, the storage was AC coupled, had 60% of the maximum output of the solar PV, and was 4 hours duration. That is from a comparison of charts ES-5 Q1 2022 with ES-1 Q1 2022.
Table of Contents
Useful additional links:-