The self-consumption strategy with storage may
have different objectives:
| - |
Consuming its own PV produced
energy, and draw a minimum of energy from the grid, whatever the
price. |
| - |
Optimizing the cost of the
electricity. This is in the case of a high price of the electricity
from the grid, and a low price of the re-injected energy. For being
profitable, the difference should be higher than the price of
the stored energy. If the tariffs are time-dependent during the
day, this may involve a specific charging/discharging strategy,
which is not yet implemented. |
| - |
For the grid management,
re-injecting the energy when needed by the community. This is not
possible to manage in the simulation, as the real state of the grid
at each time is not known. |
In this first attempt, only the first option is
implemented: the energy is stored in the battery, as soon as
available (i.e. when the PV production overcomes the user's needs),
and is "immediately" used for satisfying the internal needs up to
the battery is empty.
In this mode, the energy of the battery is
never re-injected into the grid. Keep in mind that the User's
consumption and the grid are the same circuit. Practically, the
control device and the battery inverter should be able to modulate
the power, in order to exactly feed the user's consumption.
If we define:
| - E_Avail
|
energy available after all losses
behind the inverter: E_Avail = EoutInv - EAuxLss -
EUnavail - EAcOhmL - ETrfLss |
| - E_User
|
internal user's needs
(consumption), defined in hourly values |
| - E_Grid
|
excess PV energy, injected into
the grid |
| - EFrGrid
|
back-up energy drawn from the
grid |
| - EBatCh |
the energy stored into the
battery |
| - EBatDis |
the energy drawn from the
battery |
We have the following operating modes:
The charging or discharging are limited by the
PMaxCharge of the charger, or
the PMaxDischarge of the
battery inverter.
Sizing
For a domestic-like installation:
| - |
First evaluate the average daily
energy needs from the user's needs (load) profile [kWh/day]
(shown on the dialog) |
| - |
The PV array should be able to
produce this energy for most of the year's days. |
| - |
For an "efficient" independence
on the grid, the battery pack should store at least one to 2 days
of consumption, ideally 3 or 4. |
Now there
are some additional constraints to this sizing:
| - |
The battery charging should not
be too quick: for Lead-acid batteries, a charge in 3 hours is the
minimum reasonable for the lifetime of the battery. Li-Ion
batteries support higher currents (up to 1 hour). This should be
limited by the charger maximum power. The eventual excess power
energy will be injected into the grid. If this is not desired, the
maximum PV power [kWp] should be able to charge the battery in 3-5
hours minimum. |
| - |
The inverter maximum power should
be sufficient for feeding the maximum power required by the user.
Again, the maximum discharge current should not be excessive (C3,
i.e. discharge in 3 hours, for lead-acid, around C1 for Li-ion). If
these powers are exceeded, the battery capacity should be
increased. This may be the case when the full consumption of the
day is concentrated on a short time. |
Simulation
and results
After the simulation, the balances of all these
energy flux will appear on the loss diagram:
The diagram shows:
-
The amount of stored energy (with respect to direct use),
which has an impact on the cycling, i.e. the battery lifetime and
cost of the stored energy. This is highly related to the user's
needs profile, i.e. if the energy is mainly consumed during the
solar availability or not. This may be improved by a severe
demand-side energy management (DSM), i.e. switching the appliances
from night to specific periods of the day.
| - |
EBatDis - EBatCh: The
battery energy loss due to the charging/discharging coulombic
efficiency, internal resistance, eventual gassing (lead-acid) or
over-current due to overcharging, etc. |
| - CL_Chrg |
The operating losses in the
charger (inefficiency). |
| - CL_InvB
|
The operating losses in the
inverter (inefficiency). |
| - E_User |
the total energy consumed by the
user (i.e. the specified hourly load profile), which is split
into: |
| - E_Solar |
the user's consumption coming
from the sun |
| - EFrGrid |
the missing energy, drawn from
the grid when the PV is not sufficient (especially by night) |
| - SolFrac |
we name "Solar Fraction" the
ratio of the user's energy from solar, to the total user's
consumption. |
| - E_Grid |
the excess energy, injected into the grid.
This is essentially related to the PV array power / User's
consumption ratio. |
| - E_Unused |
is the same energy quantity as
E_Grid above, when the reinjection into the grid is not allowed
(this will be implemented soon). |
The
relative values of the energies in this final balances are highly
dependent on the sizing. This result should be a guide for
optimizing the system according to your criteria.
