Battery Storage Solutions
Since 2010, we have been on a quest looking at different types of battery systems for use with PV systems as it seems to make much more sense storing any excess energy generated throughout the day for use at night.
This is a particularly tough nut to crack. EnergyZone installed the very first battery hybrid system in the UK back in 2010, under the EZsolar brand, using a Nedap inverter and Rolls lead acid batteries and whilst this has worked very well, the cost and life expectancy of the battery system plus the losses in the conversion to and from DC means that the system will need replacing before it has paid for itself. We have looked at various other systems and put together a specification of what a solar storage system should look like and perform and importantly to have a reasonable storage capacity, life and cost.
Do I really want a storage solution?
Firstly not all systems are suitable to have a battery system installed; if the system is fairly small (2-3 Kw) and the occupants are at home most of the time then there is unlikely to be much spare capacity from the PV and so a battery system would be wasted. We need to be realistic as to how much energy we can efficiently store; for example, high load devices such as cookers, heatpumps, immersions for example, will all quickly drain the energy from the battery (unless you have a very large pack with a very high discharge current capability which is un-economic). Due to this, it makes sense to size the storage appropriately meaning that it can be fully charged on most days and fully discharged through the evening. Oversizing the storage means it will not be charged sufficiently every day and so the amount of storage is not used. To strike the best balance between the energy generated and energy used is to have a storage capacity between 1 to 2 times the capacity of the array; therefore, a 4 Kw array should have a minimum usable capacity of 4Kwh and a maximum of 8Kwh. Unfortunately a lot of the battery systems on the market are only rated at 2 Kwh usable storage or less. Even with a battery system it is still a good idea to run your programmable loads (such as dishwashers and washing machines) during the daytime to make direct use of the solar energy and leave the battery to provide the low energy demand (lights, fridges, freezers etc) during the night.
Direct DC storage
Most battery storage solutions available today convert excess AC electricity into DC which in is used to charge the batteries. When there is excess demand above generation, this process is reversed converting DC back into AC. This double conversion is wasteful and will have a detrimental effect on the performance and financial payback on the system. The solution we are proposing is based around a direct storage of excess DC generated from the panels before it reaches the existing system inverter, now there is only one highly efficient DC to DC conversion taking place and the existing system inverter is used to convert this back to AC as normal.
Battery technology has come on a lot in the last 18 months and now we can offer an affordable Lithium ion battery that has a long life with 80% DoD (Depth of Discharge), for example, a battery rated at 5Kw is only ever discharged to 80% and so is using 4Kw of storage. This compares to lead acid batteries that should only be discharged to around 50%.
The other issue we have noted during our trials of battery systems is that short cycle charging of the battery reduces it’s life and effectiveness leading us to source a system that takes proper care of the batteries. It is worth noting that all batteries have limited charge and discharge rates and Lithium again does much better here than lead acid or gel batteries.
Simple installation and retrofit-able
Any solution needs to be one that can be easily fitted into most households onto existing PV installations, this means that the equipment needs to be installed near to the inverter in a safe place for the batteries and where we can connect a sensor to the incoming mains supply. This last point may be difficult to achieve as the inverter and main incomer are not normally near each other.
Here at EnergyZone we have set ourselves a high standard and have been working with suppliers over the last year or so to offer a couple of options that will reach our specification.
Growatt SP2000 System
This system is available now with a wired current sensor and in June with a wireless version. It consists of a DC charge controller connected to a 5Kwh Lithium battery system (4Kwh usable) and operates from a single string of PV. If you have 4 kw these are likely to be split between two strings, we would divert one of these to provide electricity directly to the SP2000 whilst the other continues on to the inverter as normal. Therefore, during the day, half of the energy is used to charge the batteries (if not required by the house load) whilst the other half is used to run the house. A 4 Kw array is likely to charge the battery fully in around 3 hours on a good solar day.
Solaredge and Tesla
The second option we have is based upon exactly the same type of principles we have outlined but with the option of a larger system at 7 Kw (usable, 10Kw battery).
The Tesla Powerwall battery system will provide 6.4Kwh of storage capacity, this can be charged or discharged at a maximum rate of 3.3Kw, so the system could run a 200W lighting circuit for 32 hours or a large refrigerator for 4 days. The DC version of the system will not work in a power cut but the AC version can have some dedicated loads which could continue to operate during a power cut. The warranty is for 10 years based upon a full cycle per day, although you can equally have several partial cycles per day with no detriment to the battery. Unlike some battery systems this does not use any mains electric power for maintenance charges etc. The roundtrip efficiency (charging and discharging the battery) is better than 92% so there is a potential loss of 8% from the generated power that goes through the battery, this would lead to a slight drop in the feed in tariff, typically around 100 units per year.
The final option is also becoming available towards the end of 2016 and will be a complete hybrid inverter suitable for new installations (or the inverter needs to be replaced in existing ones) and has the options of 2.4 to 12 kWh of Lithium storage, pricing details on this are still to be finalised.