Stand-alone Hybrid Energy Systems
A stand-alone power system (SAPS or SPS) (see Figure 6.4), also known as remote area power supply (RAPS), is an off-the-grid electricity system for locations that are not fitted with an electricity distribution system. Typical SAPS include one or more methods of electricity generation, energy storage, and regulation.
Electricity is typically generated by one or more of the following methods along with energy storage [116, 117]:
- • Photovoltaic system using solar panels
- • Wind turbine
- • Geothermal source
- • Micro combined heat and power
- • Microhydro
- • Diesel or biofuel generator
- • Thermoelectric generator (TEGs)
FIGURE 6.4 Schematics of a stand-alone hybrid system [115, 116, 117].
Storage is typically implemented as a battery bank, but other solutions exist including fuel cells. Power drawn directly from the battery will be direct current extra low voltage (DC ELV), and this is used especially for lighting as well as for DC appliances. An inverter is used to generate AC low voltage, which can be used with more typical appliances. Stand-alone photovoltaic power systems are independent of the utility grid and may use solar panels only or may be used in conjunction with a diesel generator, a wind turbine or batteries. The two types of stand-alone photovoltaic power systems are direct-coupled system without batteries and stand-alone system with batteries .
The basic model of a direct-coupled system consists of a solar panel connected directly to a DC load. As there are no battery banks in this setup, energy is not stored, and hence it is capable of powering common appliances like fans, pumps, etc. only during the day. MPPTs are generally used to efficiently utilize the sun’s energy, especially for electrical loads like positive-displacement water pumps. Impedance matching is also considered as a design criterion in direct-coupled systems. The hybrid system is preferred over direct-coupled system because it provides better quality and stable power.
In stand-alone photovoltaic hybrid power systems, the electrical energy produced by the photovoltaic panels cannot always be used directly (see Figure 6.5). As the
FIGURE 6.5 Schematic of a stand-alone PV system with battery and charger .
demand from the load does not always equal the solar panel capacity, battery banks are generally used. The primary functions of a storage battery in a stand-alone PV system are [115, 116]:
- • Energy storage capacity and autonomy: To store energy when there is an excess available and to provide it when required.
- • Voltage and current stabilization: To provide stable current and voltage by eradicating transients.
- • Supply surge currents: To provide surge currents to loads, like motors, when required .
In general, the hybrid power plant is a complete electrical power supply system that can be easily configured to meet a broad range of remote power needs. There are three basic elements to the system—the power source, the battery, and the power management center. Sources for hybrid power include wind turbines, diesel engine generators, thermoelectric generators, and solar PV systems. The battery allows autonomous operation by compensating for the difference between power production and use. The power management center regulates power production from each of the sources, controls power use by classifying loads, and protects the battery from service extremes.
Monitoring photovoltaic systems can provide useful information about their operation and what should be done to improve performance, but if the data are not reported properly, the effort is wasted. To be helpful, a monitoring report must provide information on the relevant aspects of the operation in terms that are easily understood by a third party. Appropriate performance parameters need to be selected, and their values consistently updated with each new issue of the report. In some cases it may be beneficial to monitor the performance of individual components in order to refine and improve system performance, or be alerted to loss of performance in time for preventative action. For example, monitoring battery charge/discharge profiles will signal when replacement is due before the downtime from system failure is experienced . IEC (International Electrotechnical Commission) has provided a set of monitoring standards called the “Standard for Photovoltaic system performance monitoring” (IEC 61724). It focuses on the photovoltaic system’s electrical performance, and it does not address hybrids or prescribe a method for ensuring that performance assessments are equitable .
Performance assessment involves:
- • Data collection, which is a straightforward process of measuring parameters.
- • Evaluation of that data in a manner that provides useful information.
- • Dissemination of useful information to the end user.
The wide range of load-related problems identified are classified into the following types:
- • Wrong selection: Some loads cannot be used with stand-alone PV systems.
- • House wiring: Inadequate or low-quality wiring and protection devices can affect the system’s response.
- • Low efficiency: Low-efficiency loads may increase energy consumption.
- • Stand-by loads: Stand-by mode of some loads waste energy.
- • Start-up: High-current drawn by some loads during start-up. Current spikes during the start-up can overload the system temporarily.
- • Reactive power: The circulating current can differ from the current consumed when capacitive or inductive loads are used.
- • Harmonic distortion: Nonlinear loads may create distortion of the inverter waveform.
- • Mismatch between load and inverter size: When a higher rated inverter is used for a lower capacity load, overall efficiency is reduced
GE Stand-alone Modular, Containerized Digitally Connected Hybrid Power Solution for Off-grid Electrification
According to their website publication , GE hybrid containerized power unit is designed to provide reliable electricity in off-grid applications, such as village electrification or remote commercial and industrial operations. The unit can also act as a backup to unreliable or inadequate grid supply. Pre-wired and configured, the hybrid energy power unit can be installed and commissioned within hours upon arriving at site. Its hybrid controller ensures that the lower cost power source is prioritized, automatically controlling the available sources to ensure that demand is met. Its remote monitoring and diagnostic capability eliminates the need for local supervision and control. The modular hybrid power unit also enables paralleling of multiple units, helping supply easily to adjust with changing demand.
A 20-foot enclosure with pre-configured diesel generator, energy storage, protection and control equipment, and power electronics can be connected to the distribution lines to begin generating power. According to their publication , the unit carries the following features:
- • Easy external DC connection point for solar arrays
- • Hybrid controller reduces operating costs by up to 40% when compared with traditional diesel generators
- • Cloud-enabled monitoring and diagnostics using Predix™ allows for remote supervision and control across multiple installations
- • Access doors to key components allowing easy service
- • Temperature and moisture-controlled compartments ensure components remain at optimal operating conditions
- • Quickly increase capacity either by paralleling multiple systems or adding incremental solar and energy storage
- • Optional equipment includes grid-tied transfer switch, weather monitoring—integrated into the solution’s remote monitoring system—as well as interior/exterior lighting
Benefits of the system include:
- • Lower installation and commissioning time and expenditures
- • Reduced operating cost and emissions versus diesel systems
- • Quickly scale output to capture growing demand
- • Achieve higher uptime, identifying issues before they cause unplanned downtime
- • Enhanced ability to monitor and control multiple installations
- • Operate reliably through a variety of environmental conditions
GE’s hybrid power containerized unit for off-grid applications focuses on quality service to remote areas, supported by local service capability to facilitate increased uptime. The picture and the detailed characterization of various components of GE system are described in their website publication. There has been an extensive investigation on off-grid energy systems including stand-alone power systems for rural electrification over the past decade. This is largely driven by the need for supplying electricity to rural, remote, and poor areas of the world. The global landscape of off- grid energy systems is addressed in Section 6.7 of this chapter.