Can I include a battery in a grid-connected system ?

[André Mermoud]

In the present time the Grid-systems and the Stand-alone systems are quite distinct in PVsyst.

Grid systems with a battery storage are more and more used now in the reality.

However their simulation is a very difficult problem, not yet implemented in PVsyst.

I don't see any possibility for approaching a solution with the existing Grid-connected and Stand-alone systems presently available in the software.

The objective of such hybrid systems may be quite different from case to case:

- For "purists" of the PV energy, consuming a minimum of energy coming from the grid, whatever the price,

- According to consuming and feed-in tariffs, optimizing the costs of electricity,

- For the optimization of the grid management, injecting power during the "best" periods of the day,

- Grid management: peak shaving,

- Grid management: short term grid stabilization (for example for clouds),

- In "rich" countries, ensuring a secure back-up in case of (rare) grid failure,

- In countries where the grid is weak or intermittent, ensuring electrical availability during the whole day,

- Mini-grids for the electrification of whole villages or islands,

- etc...

Each of these uses of the PV energy will involve different sizings, different constraints, and quite different control strategies.

On the one hand, the control will depend on the self-consumption profile and the grid characteristics (availability, overload, etc),

On the other hand, when should the PV array charge the batteries? When they are not full ? When the consumers are low? When the foreseen weather of the next day is bad ?

And how to optimize the size of the batteries ? As an order of magnitude: for an household consumption of 15 kWh/day (a standard in Europe), storing one only day consumption would represent about 700 Ah for a 24V battery bank, i.e. about 600 kg of Lead-acid batteries.

Remember that the price of the stored energy is very high. It can be evaluated by the price of the battery pack, divided by the total energy stored along the battery lifetime, i.e. Capacity (in kWh) x DOD x Max. nb. of cycles. If you assume a full storage/destorage every day, a battery pack of 1'500 cycles should be replaced every 4 years. For household systems connected to the grid, this price of kWh should be compared to the difference between the buying and the selling prices.

Now components manufacturers propose a great variety of devices, configurations, usually specifically suited for one or the other of these uses.

In this evaluation, we should also define the prices of the injection, consumption, stored energy (taking the limited lifetime in terms of number of cycles into account).

Many people ask for such systems and strategies, without understanding that the problematics is very complex and the realization expensive.

We intend to study these systems during the next months. We will probably propose simulations for a selection of these kinds of systems, to be extended progressively.