Stand-Alone Power Supply (SPS) System Module 4

Storing Energy

One way of managing the variability associated with solar and wind energy is to store the energy.

Energy Storage

Wind and sunshine are not always available when there is a demand for energy, so backup storage may be required when using renewable inputs especially in SPS systems. The energy is usually stored in deep cycle lead-acid batteries that are similar to car batteries, but are better suited to the heavy charging and discharging typical of SPS systems. Although there are a large number of batteries available on the market for a variety of applications, Lead- Acid batteries are still the best technology available for storing energy from renewable sources in SPS systems.

The Battery Bank

When several batteries are connected, they are called a battery bank. Typical system voltages are 12, 24, 48, 96, 110 & 120 Volts DC. These can consist of many individual batteries either 12, 6 or 2 volts connected in series. In SPS systems, a number of batteries are required to store the energy produced by the solar or wind system. There are a number of different voltages which can be used in systems and it is important to know what voltage is being designed for your system.

Figure 1 A typical battery bank

Battery Terminology

Battery ratings are expressed using the voltage and the storage capacity of the battery. The storage capacity of a battery is the quantity of electricity that a fully charged battery can deliver under specified conditions. This capacity is expressed in Ampere-hours (Ah) and is usually specified at 25°C. It is determined by the time taken to discharge the battery at a constant current until a specified cut-off or final voltage is reached, which is dependent on the battery type and manufacturer. The capacity is stated as an Ampere-hour value at a discharge rate, which is noted by a "C" followed by a number indicating the rated hours. An example of a typical rating is 2 volt battery with a capacity of 100 Ah @C100 rate.

Figure 2 Battery Information is often located on the battery itself

Battery Types

Either vented cell (wet cell) or sealed cell (gel cell) batteries can also be used to store electricity generated by renewable energy conversion devices. Batteries in a SPS system must have good performance at low and high temperatures, a long cycle life in deep discharge applications and a high energy density. In sealed cell batteries there is a minimum amount of electrolyte absorbed in a gel. These batteries have the advantage of being low maintenance as compared to traditional wet cell batteries, as there is no need to add water.

Figure 3 Batteries can come in many shapes, sizes, ratings, and colours.

Battery Charge Controllers

To protect the battery bank from over-charging and over-discharging, a battery charge controller should be used. The simplest method used for charge control will turn off the energy source as the battery voltage reaches a maximum and will turn off the load when the battery voltage reaches a preset minimum. The battery charge controller for a system is more commonly referred to as a regulator. Battery charge controllers prevent the batteries from over charging and over discharging. The controller may be associated with other parts of the system or may be a separate component. Some battery charge controllers may also be combined with communications technologies, where satellite phone links, on site cameras and the like are used for control and monitoring. There can also be a degree of control via direct radio links. There are 3 main types of regulator; Shunt, Series and Pulse Width Modulated (PWM) regulators.

Shunt Regulators

When the batteries are fully charged the power from the renewable source is dissipated across a dump load. These are commonly used with wind turbines. Some systems also include a Shunt Regulator which diverts energy generated, typically from a wind system, to a dump load. The dump load uses the excess energy generated by the turbine and uses it, typically to generate heat.

Figure 4 A wind turbine often uses a shunt regulator and a dump load to dump the excess electricity that is produced when it cannot be used or stored.

Series Regulators

When the batteries are fully charged the power from the renewable source will be switched off in the simplest series regulator. An improvement for this type is the proportional type, where the current is controlled by a variable component in series with the renewable source.

Figure 5 A series regulator

PWM Regulators

These regulator switches the control device on and off quickly. When the batteries are discharged the unit will be switched fully on. As the battery is reaching a fully charged state the unit will start switching the control device on and off in proportion to level of charging required. When the battery is fully charged no current will be allowed to flow to the battery. PWM regulators are usually used with solar modules.

Inverters

The inverter enables the use of standard appliances in SPS systems. An inverter is an electrical device that changes direct current (DC) into alternating current (AC). Inverters often incorporate extra electronic circuits that control battery charging and load management. Generally, inverters used in most household systems now produce power of a similar quality to that in the main electricity grids, these are referred to as true sinewave inverters. Early model inverters produced lower quality power, which was adequate for most appliances. These are now only used in very small inexpensive systems. They are often referred to as modified square wave inverters and sometimes as modified sinewave inverters.

Renewable energy systems often provide low voltage, direct current (DC) from batteries, solar panels or wind generators. To use this DC power directly requires special non-standard appliances that may be available for camping and other portable or low power applications. Some appliances, such as fridges are relatively expensive. Electricity available from the main electricity grid is provided as alternating current (AC) at 240V in Australia, so most appliances are manufactured to suit this supply. The electrical energy used by the appliance is referred to as the load on the system.

Figure 6 An inverter

An inverter is an electrical device that changes direct current (DC) into alternating current (AC). The inverter enables the use of standard appliances in SPS systems. Inverters often incorporate extra electronic circuits that control battery charging and load management. Generally, Inverters used in most household systems now produce power of a similar quality to that in the main electricity grids, these are referred to as true sinewave inverters. Earlier model inverters produced lower quality power, which was adequate for most appliances. These are now only used in very small inexpensive systems. They are often referred to as modified square wave inverters and sometimes as modified sinewave inverters.

Case Study – WebRAPS

A typical SPS family home would expected to use around 7kWh per day at, say, at an approximate constant rate. This system is designed to supply such a load using a hybrid PV - Wind - Battery SPS system. Therefore wind and PV's supply 100% of the load. It was installed in 1998 for approximately $42 000.

Figure 7 The WebRaps installation

Figure 8 shows the average performance of different components of the WebRaps system over the course of a typical year.

Figure 8 Component performance of WebRaps

There is more information on this installation in RISE's Demonstration Renewable Energy Systems.

Case Study – Munda Station

Munda Station is a working pastoral station with a large seasonal load variation.

Dry season - 39.8 kWh/day
Wet season - 107 kWh/day

It is a hybrid system (PV – Diesel – Battery) of which PV supplies between 37 – 80% of the load. It was installed in 1999 at a cost of approximately $120 000. The design brief for this system was to provide a hybrid diesel SPS system capable of supplying power to the remote cattle station, Mundabulunga. Commonly known as Munda Station the 225,000 hectare cattle station is located approximately 100 km south-west of Port Hedland. The prime activity of this station is the breeding and exporting of cattle for the south-east Asian market. Three or four fulltime staff are on site all year round, rising to twenty during periods of peak activity. Munda Station itself comprises six main buildings and several workshops and storage sheds.

Figure 9 the arrays at Munda Station

On average the PV panels at Munda supply 80 % of the required power during the dry season, falling to 37 % during the wet season, with an overall average of 48 %. The diesel generator is mainly required in the late afternoon and early evening where it is meeting the stations base load energy demand plus charging the battery bank. The diesel also self-starts at various periods throughout the day in order to maintain the battery state of charge. This is quite uneconomical and better load management could be used to minimise its occurrence.

This is the end of Module 4.

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