Context
More and more grid-tied PV systems are now
equipped with a battery storage.
The objective of such hybrid systems may be
quite different from case to case. As examples:
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For "purists" of the PV energy,
consuming a minimum of energy coming from the grid, whatever the
price, |
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According to consuming and
feed-in tariffs, optimizing the costs of electricity, |
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For the optimization of the grid
management, injecting power during the "best" periods of the
day, |
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Grid management: peak
shaving, |
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Grid management: short term grid
stabilization (for example under variable cloudy conditions), |
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In "rich" countries, ensuring a
secure back-up in case of (rare) grid failure, |
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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, |
Each of
these uses of the PV energy will involve different sizings,
constraints, energy flux, 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,
the charging/discharging strategy is important. When should the PV
array charge or discharge the batteries?
| - |
When they are not full ? |
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When the consumers have low
needs? |
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According to the time-dependent
tariffs ? |
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When the foreseen weather of the
next day is bad ? |
Implementation
in PVsyst
Since the version
6.76, PVsyst provides 3 different strategies of
Grid-storage:
Each of
these strategies have different constraints:
| - |
Self-consumption and Weak grid
recovery require the definition of a user's needs hourly
profile, |
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Weak grid recovery requires the
specification of a grid-unavailability hourly profile, |
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Weak grid recovery may accept
re-injection of PV energy into the grid, or not, |
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Peak shaving doesn't involve a
user's needs profile, |
| - |
The battery energy will never be
used for feeding the grid, except with peak shaving, |
| - |
In all these strategies, the
battery charging will begin as soon as PV energy is over the user's
needs. |
| - |
The time of release of the
battery energy (discharge) may be different according to the
strategies, cost optimizations, etc. |
The sizing of
the different parts of the system (PV array, battery pack, as
function of the needs profile and the electricity price), is a
complex problem, specific to each of these strategies. PVsyst will
probably provide only rough sizing rules until some experience has
been accumulated.
Real System
realization
Grid-storage
systems require specific electronic devices, especially suited
inverters, battery chargers, controllers, etc.
Defining these
devices in PVsyst will be extremely complex, as each manufacturer
proposes its own integrated solution.
Moreover some
battery packs for domestic use are delivered with AC inputs and
outputs, meaning that a charger and an inverter is included in the
pack.
In this first
attempt, these specific devices are not yet implemented. The
charging and discharging operations of the battery are approximated
by generic charging (AC-DC) and discharging (DC-AC) devices,
characterized by an efficiency curve as function of the power, and
a maximum output power PNom. These give rise to a conversion loss
in the loss diagram.
Cost of
energy
Implementing a storage in a PV system implies an specific cost of
the stored energy, expressed as price/kWh.
This cost
corresponds indeed to the maximum energy stored in the battery pack
during the battery lifetime, divided by the cost of the battery
pack replacement.
The simulation
will calculate the Battery ageing as a function of the operating
conditions (nb. of cycles and temperature), and evaluate its
degradation along the time.
The simulation
will perform the replacement of the battery pack when necessary.
This should be a crucial information for the financial evaluation
and optimization, as well as for the ageing of the whole
system.
