We intentionally left this subject to the end of this chapter, because the batteries do not generate energy, they just store its surplus for times when the demand is higher than supply. As such, they are a crucial part of any off-grid system. It’s in sharp contrast to solar systems deployed in houses connected to the grid.
They do not need storage batteries. It’s because the excess solar power (if any) is instantly sold to the grid, and the deficit of energy (if any) is instantly supplied by the grid. In a way, the grid plays the role of the energy storage with an infinite capacity. This is a continuous, automatic process that you can monitor, although practically you would rather see its outcome (balance) only on your monthly energy bill.
In any off-grid location, you will not have such luxury. First of all, the highest amount of solar energy is generated at midday while its highest consumption occurs starting late afternoon and continues till early night. Successive rainy or cloudy days will make it worse as the solar system will generate very little energy.
The wind-generated energy is even less predictable (well, shore locations may have more consistent wind patterns). That’s why any well-designed off-grid solar or wind power system must have higher installed power than the daily energy needs and higher capacity of storage to be able to store the surplus of energy for times when generated power is lower than the demand or simply not available at all. The amount of required “overcapacity” is a compromise between the cost, weather statistics for a given zone and required comfort (continuous availability of energy).
In solar systems there is significant shift between periods of high-power consumption and high-power generation. Source: EnergySage.com
- The most important parameters characterizing batteries for off-grid applications are:
1. Nominal Voltage
The most popular nominal voltages are 12V, 24V and 48V (we exclude here all sort of small batteries designed for electronic devices). Note that individual batteries can be connected in series (two 12V batteries will provide 24Vdc), or in parallel (two 12V batteries of 100Ah capacity each will make the 12V unit with capacity of 200Ah).
For practical reasons, most subsequent parameters are specified in respect to current (Amperes) rather than power (Watts). Knowing the nominal voltage of a given battery, it is easy to calculate the Power, while knowing also the time, the stored or delivered Energy.
We should underline the fact that the actual voltage measured at the battery will slightly differ from the nominal value. It will be higher than nominal when fully charged and continuously decreasing with discharge.
Typically, it is specified in Ah (Ampere-hours), indirectly however it indicates the maximum amount of energy the battery can store. Theoretically, the fully loaded 12V battery with capacity of 100Ah should be able to supply the 1A current over a period of 100 hours, what will be equal to 1.2kWh of stored energy (12V x 1A x 100 hrs), but it can be as well 10A over the period of 10 hours.
3. Power Rating
Specified in Amperes (A), it represents the maximum current that can be supplied at any given time by the battery. One would assume that the fully loaded 12V battery with the capacity of 100Ah may also supply 100A current over a period of 1 hour, but this is a False assumption. Prolonged supply of current higher than the rating will shorten the battery life.
4. Depth of Discharge (DoD)
While each battery has specified capacity (in other words the amount of energy it can store), it does not mean that all stored energy will be available for the user. The mentioned earlier examples are theoretical, because batteries should not be emptied to zero as it will badly affect their reliability and future performance. The maximum level of discharge is specified by the Depth of Discharge (DoD) and is expressed in % of Capacity. For example, the DoD of 50% means that the battery can be discharged to 50% of its initial capacity.
Batteries used in off-grid systems (this also includes RV applications) are designed to supply limited power for a long period of time and be able to deliver most of its stored energy. To serve this goal, they must have the highest possible Depth of Discharge to guarantee the availability of the stored energy but at the same time to not jeopardize their longevity (in other words good performance over long years of service). Such batteries belong to the class of Deep Cycle batteries and usually can be discharged down to 20% of their specified capacity (corresponding to the DoDs of 80%). It is worth mentioning that deeper discharges have a negative impact on the battery’s lifespan.
In contrast, automotive batteries do not really face such situations. They are specifically designed to supply short-time high-peak currents required to start the engine. Practically, such events last for just a few seconds, so the battery supplies only a small fraction of its stored energy and then is quickly re-charged by car’s generator. For obvious reasons, they are commonly known as Starter Batteries.
Also known as Round-Trip (RT) efficiency, it expresses the fact that when you feed the battery with an energy of 1kWh, you will not be able to fully get it back. Part of that energy will be lost (converted to heat during charging process and discharging process). If the efficiency of a given battery is specified as 90%, it means that you can get back only 0.9kWh from the mentioned 1kWh. To make it even worse, the Round-Trip efficiency describes the instant value right after charging. With time the battery will be losing its stored electrical energy due to internal “leaks” (discharge). So, for example after a few days the available energy will be lower than 0.9kWh and after longer time it will go to zero. The internal discharge is highly affected by the ambient temperatures.
Note that RT Efficiency has nothing to do with availability of stored energy determined by the DoD. Discharging the battery to let’s say 20% of its initial capacity does not mean that the battery is emptied. It still stores some amount of energy (theoretically 20% of its capacity), but it is unwise to use it as further discharge will shorten battery’s life.
6. Cycle life
Each class of batteries has determined (and unfortunately limited) number of charge-discharge cycles that it can experience without losing specified performance. Popular lead-acid batteries have cycle life in the range of 1,000 to 2,000, while modern Lithium-Ion batteries have specified cycle life of about 5,000. The real number of cycles depends on “working conditions” (rate of charges, discharges, temperatures etc… ). With time the battery will be losing its performance, especially its capacity (in other words it will not be able to store the specified amount of energy) and efficiency (less stored energy available to the user).
- 1. Lead-acid are the most popular batteries available on the market today. They come in 3 versions:
a) “Flooded” (wet) batteries.
They require maintenance (mainly adding distilled water) and ventilation due to gassing. They are low cost, but heavy, bulky and corrosive. This technology is used for over 150 years, so it is well-matured, but due to the mentioned problems they are rarely used these days.
b) Sealed batteries.
They come in two versions: AGM and Gel. Both versions are sealed, so they do not need maintenance or ventilation.
They have better performance than flooded batteries: can tolerate deeper DoD cycles (80% or even more, especially Gel versions), are lighter, have a longer lifespan but all that comes at an extra cost. While AGM batteries are slightly more expensive than flooded ones, Gel batteries are significantly more expensive.
It’s worth to note that all lead-acid batteries are fully (or almost fully) recyclable.
This is relatively new technology, developed in the 80’s and now widely used in all portable electronic devices (laptops, tablets, phones etc..). Recently however, with the arrival of hybrid and all-electric cars, the lithium-ion technology also found its way into industrial applications requiring much larger capacities.
The Lithium-Ion batteries have much better Capacity -to- Volume ratio than lead-acid ones, so they are suitable for stacking to achieve large storage capacity. They are sealed (so do not need maintenance), have excellent DoD values (reaching 100%) and large number of life cycles (up to 5,000) – in other words they offer all that is necessary in off-grid systems. The only problem seems to be their cost, still few times higher than traditional lead-acid models. However, with the observed explosion of applications in auto-industry and mass production we may expect that prices will go down.
3. Cost and configuration
Sizable bank of batteries for off-grid applications is quite expensive (depending on batteries it may count for about half of the total cost of the solar or wind-powered system).
a) While Lithium-Ion batteries are still quite expensive, given their favorable characteristics for off-grid applications and long lifetime, they may still prove to be more economical than significantly less expensive lead-acid batteries.
b) The most popular (and less expensive) are 12V batteries, however it does not mean that they offer the best solution for the Off-Grid Network. In higher power applications, the system will face high currents requiring adequate gauge wiring, almost perfect interconnections and regardless of all these efforts will still incur substantial losses (power loss is proportional to the square of the flowing current). For example, at 12V, a load of 1.2 kW will require 100A current. At such high amplitudes any, even the smallest “imperfection” in wiring will be the source of heat and potentially can cause the fire.
By increasing the nominal voltage to 24Vdc, the current (for the same power) will be halved while thermal losses cut by 4. Using 48V network will further improve the situation bringing the current down to ¼ and power loss to 1/16 of its respective values at 12V.
c) For practical reasons (cost and availability), it may be less expensive to build the 24V system by connecting two 12V batteries in series than buying one 24V battery and certainly less expensive than buying 48V one. It’s not that higher voltage batteries are more costly in production; it is just the law of big numbers. The market is flooded with 12V batteries, 24V models are also available because they are used for trucks (although mostly, these will be lead-acid versions). The 48V models are used as backup sources for security, in telecom centers etc.. so their price will not share benefits of mass production.
LG Chem RESU 13 – a 48Vdc lithium-Ion battery. With its 13.1 kWh gross capacity (usable energy 12.4 kWh), DC round-trip efficiency of 95%, rated to 6000 cycles with durability ensuring 80% of capacity retention after 10 years and light weight, the battery represents the state-of-the-art storage solution. It’s powerful, elegant but $$$.
d) Looking at the efficiency of the system is only one part of the job. You may have to also check if required appliances are available at these voltages so all of them (or at least those power hungry) can run at the higher than 12Vdc nominal voltage.
Performance of renewable off-grid energy systems is affected by both – power generating components (PV panels and/or turbines) and power storing components (batteries). You may have substantial surplus of generated energy, but if you cannot store it, or if the batteries’ efficiency drops down, you won’t have it fully available when needed. That’s why it is highly important that batteries are properly maintained and not “abused” during their lifetime operation, preventing this way their accelerated degradation.
Usually, the charging process is regulated by the Control & Monitoring System. It must be “tuned” to the given models of batteries and “smart enough” to constantly monitor the state of batteries and make required adjustments (tight control is especially important for Lithium-based batteries).
a) Note that overcharging the battery is us bad as fully discharging it (below DoD levels) or leaving them discharged.
b) Similarly, negative impact on the battery life has higher than specified rate of charging and discharging. While the Monitoring & Control system is expected to keep charging rate within the specs of batteries, the discharge rate will fully depend on your loads. Note that fridge and AC unit draw surge current (so-called Startup current) every time the compressor turns on. Its amplitude can be 4-to-5 time higher than the specified nominal value. Such dynamic loads are not well tolerated by most of deep-cycle batteries.
c) High temperatures (either ambient or from overheating) significantly shorten the battery life. It is expected that the Control & Monitoring system will prevent overheating, it is up to you to prevent expose of batteries to the sun (or any other source of heat).
Bottom line: – carefully read batteries’ instructions before connecting them to your off-grid power system. Keep in mind that battery’s operating conditions have dramatic impact on its performance and lifespan.
All what you read above regarding potential sources of energy in off-grid environment is just an introduction to give you some ideas. However, given the complexity and cost of any practical solution you should consider it as an invitation to start deeper research on the subject focusing on solutions suitable for your location and acceptable for wallet.