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Small Scale Electricity Generating Plant

(Renewable Energy Opportunities)

 

Alternative Energy Supply Installations

The most cost effective way to generate electricity is with very large power plant because of the economies of scale. This however is not always possible or even desirable. By concentrating the power generating capacity in fewer but larger plants, the distribution cost of reaching the outlying customers increases as well as the electrical distribution losses in the long power lines. For some remote locations it may not be practical or economically viable to provide a connection to the distribution grid system. Installations making use of renewable energy resources must be located where the resource is available and by their nature, the size of these installations is limited by the magnitude of the available energy flow.

The complexity of small scale systems is determined by the nature of the energy supply and by whether they are connected to the grid or whether they are stand alone systems.

 

Capturing Renewable Energy

Many small scale installations are designed to take advantage of renewable energy. They depend on capturing naturally occurring energy flows from the elements which are both intermittent and notoriously unpredictable as well as being liable to wide variations in their intensity when the energy is available.

At the same time, because the number of consumers is small, the load too is likely to be subject to wide swings in demand. Furthermore, although the energy may be available, the small scale of the operations limits the average amount of energy which can be captured and the diverse nature of the methods needed to convert it into useful electric energy both result in poor conversion efficiencies.

Very large systems are usually grid connected and supply power only when it is available, otherwise the demand is satisfied by other sources connected to the grid.

Detailed descriptions of renewable energy opportunities and applications are given in the sections on Wind Power, Solar Power, Hydroelectric Power, Geothermal Energy and Hybrid Power Systems

 

Although the energy may be free, the cost of capturing it can be prohibitive. The electrical output of the prime energy conversion source is not usually in a form which can be used directly in the consumer's equipment and additional components are required to match the supply to the load. These components are collectively known as the Balance of System (BOS)

 

  • Grid Connected Systems

    Grid connected systems are usually designed to take advantage of some locally available energy source with reserve power coming from the grid during periods of high demand. For larger systems they also provide the opportunity to sell surplus energy back to the utility company. In this case however there are strict specifications on what can be pumped back into the grid. The local generator output voltage and frequency must be maintained within tight limits and the waveform should be a pure sine wave, free from harmonics. For small domestic systems, the cost of setting up a sell back arrangement and the price offered by the utility for the small amounts of surplus energy involved usually make the sell back proposition unattractive. Since the cost of electric energy from the utility may be many times the sell back price, it makes more economic sense to store surplus energy in a battery, to avoid purchasing electric energy during consumption peaks or production shortfalls.

    Because of the grid connection, power can be supplied by the grid if the local source fails or is insufficient to meet the demand so grid connected systems do not need storage batteries or emergency back up systems. On the other hand, the local source will not necessarily be able to provide emergency power to the consumer if there is an interruption in power from the utility. If such failures occur, local power generators must be automatically disconnected from the grid so as not to give a nasty surprise to the lineman trying to fix the fault. Since the inverters providing the AC output in the local grid connected systems take their voltage and frequency references from the grid, they will also be rendered powerless when this happens. If the purpose of the local system is to provide emergency power during utility supply interruptions, it should be specified as a stand alone system.

     

  • Stand Alone (Off the Grid) Systems
    Stand alone, generating systems are needed to supply electrical power to remote locations where connection to the grid is either not possible or inconvenient or to provide a power back up in case of unreliable supplies from the power utility.

    Applications include:

    • Domestic and Small Commercial / Industrial Applications
    • This would include remote farms, holiday homes, telecommunications repeater stations and transmitters.

    • Specific Purpose Power Units
    • Simpler, dedicated power units are needed for applications such as pumping water, electric fences, traffic signals, navigational aids, cathodic protection (CP) to prevent corrosion in oil, gas and chemical installations, remote monitoring and portable systems for recreational and emergency vehicles. Many of these very small systems are designed to run on DC rather than alternating current and do not require expensive inverters and control systems.

     

    Off grid systems may be powered by diesel generators or by some locally available renewable resource such as hydro, wind or solar energy. Diesel generators usually have a controlled fuel supply and are designed to supply regulated AC power directly. Wind and solar energy are however available only intermittently and subject to wide variations. Hydro energy may be constantly available but still subject to variations. Off grid systems dependent on renewable energy therefore need back-up batteries to supply power when the prime energy supply is insufficient or unavailable. Because of the need for energy storage, small scale systems are usually designed to produce and store DC and an inverter must be used to generate the necessary regulated AC output using power from the battery.
    Photovoltaic systems generate DC directly but wind powered generating systems usually employ low cost AC generators whose output voltage and frequency vary with the speed of the wind and can not be used directly to power normal AC loads. Since the batteries can not store alternating current, the output of the generator is rectified and fed through a regulator to provide a DC link which is used to charge the battery and to power the inverter. It is also simpler to derive the regulated AC power from a fixed DC supply than it is to convert the variable AC to a regulated AC directly.

     

    Batteries are needed in off grid systems for three reasons

    • Storing energy for use when energy from the prime source (solar, wind etc.) is not available.
    • Creating a clean regulated AC supply to the consumer from an unregulated source via a regulated DC link
    • As a buffer to match intermittent supplies to peaky demands
    •  

  • Emergency (Portable) Power Generation
  • Small scale systems for emergency use are discussed in the sections on Gas Turbine and Piston Engine power plants.

     

Balance Of System (BOS) Components

To make the best use of the available resources, besides capturing the energy, additional system components, known as the Balance of System (BOS) components, are required for maximising the energy capture, for regulating the energy flow, for converting the electrical energy into the voltage and frequency suitable for the applications, for storing the surplus energy, for demand management, for providing continuity of supply and for safety systems to protect the users and their investment. Examples are shown in the diagram of the Remote Area Power Systems (RAPS).

 

  • Voltage Regulator
  • Most small generating system power electronic circuits are based around a standard DC reference voltage. This is the voltage of the so called "DC Link" between the variable power source and the regulated AC voltage supplied to the user. It is typically the system battery voltage and the operating voltage of the DC Control Unit.

    The voltage regulator takes the variable output voltage from power source, either DC as in solar voltaic systems or rectified AC from asynchronous generators such as wind turbines and provides the fixed system reference DC voltage at its output terminals to charge the battery and feed the inverter.

    Using a fixed DC level to the inverter simplifies the inverter design.

     

    See more about voltage regulators in the section about Chargers.

     

  • Storage
  • Energy storage is needed to provide power on demand from intermittent sources.

    High capacity storage batteries are used for off-grid power generating systems to be able to keep the system operational during prolonged periods when the energy demand exceeds the supply, for example when solar panels are in the dark or wind turbines are becalmed. These batteries normally suffer deep discharges and must be purpose designed to cope with the very demanding charge-discharge regime. Automotive batteries used for starting, lighting and ignition (SLI batteries) are designed to cope with shallow discharges since they are constantly topped up by the engine and are not suitable for this application.

     

  • Power Conditioning
  • The power conditioner's task is to provide pure, noise free sine wave power at a fixed frequency and voltage to the user, replicating the mains power from the grid.

    • Inverter
    • The inverter is the component which converts DC battery power to AC power at the standard utility supply voltage and frequency. Since the battery voltage is usually quite low, the inverter incorporates a DC - DC converter to transform the low voltage DC input to a higher voltage DC to supply the inverter circuit. The AC output voltage level depends on DC input voltage and unless a stable DC level is supplied by an external regulator, the inverter must also incorporate its own voltage regulator.

       

      The output from an inexpensive inverter could be more like a square wave than a sine wave and for many applications this doesn't matter, however the high harmonic content from such inverters can cause some applications to malfunction. Furthermore, any system designed to supply power back into the grid must meet the utility's very tight tolerances on wave shape, harmonic content, and frequency and voltage stability. This is the function of power conditioning which demands more complex circuit designs and better filtering of the output.

       

      • Grid connected systems
      • Any system connected to the grid must be synchronised with the grid. They therefore use synchronous inverters which take their frequency and voltage references from the grid.

        Small scale systems not managed by the utility must disconnect if grid fails as noted above so that maintenance workers repairing an out of service power line are not surprised by rogue voltages on the line. Unless alternative power and frequency references are available the system will switch off automatically.

         

      • Stand alone systems
      • Inverters used in off-grid systems must generate their own frequency standard.

         

      See more about inverters in the section on Electric Drives and AC Batteries.

       

    • Load Demand Management
    • Demand management is only necessary in stand alone systems. It is a way of making capital cost savings on expensive generating equipment at the expense of inconvenience to the user. It may also be important for installations which only have access to limited power sources.

      When the demand for energy exceeds the supply, the power to low priority loads is switched off under microprocessor control based on a predetermined set of priorities.

      This is also known as load shedding

       

    • Load Diversion - Energy Dump
    • This is the converse of load shedding. When the energy supply exceeds the demand the energy must somehow be dumped.

      When the battery is fully charged and the system is still capable of generating surplus energy, in theory the generator could simply be switched off or disconnected, however this is not the ideal solution. In the case of a turbine, suddenly removing the electrical load could cause the turbine to overspeed unless automatic braking is applied. It makes more sense to provide an alternative "dump load" to store or use the surplus energy rather than to waste it. The simplest solution is to use the surplus energy to heat water either for the domestic hot water supply or for space heating.

       

  • Safety
  • As we might expect in any mains electrical installation, safety is a high priority and the system must be provided with fuses, circuit breakers and surge suppressors to protect both the users and the equipment.

     

  • Buildings and Installation Materials
  • BOS costs also include mounting racks, instrumentation, switchgear, hardware for the panels and all the electrical wiring as well as the installation costs for the buildings and generating equipment.

     

  • Emergency Power
  • Many off grid systems incorporate a stand-by diesel electric generator to provide emergency power in case of unusual conditions or events.

 

Sizing Components for Small Scale Systems

System dimensioning involves matching the scale of the energy source to the expected user demand, starting with the user's consumption pattern and working back towards the source. This is to take into account the Balance of System losses which can be very high.

 

  • Grid Connected Systems
  • There are few constraints when sizing grid connected systems. They can be as large or as small as you like. Surplus energy from large systems can in theory be sold to the utility company. Energy from the grid can make up for energy shortfalls or satisfy peak demands when small systems can not carry the load. Thus it is less important that the local system is precisely matched to the expected demand and energy storage is not necessary.

    Grid connected systems are typically sized take the maximum advantage of the available power, provided a satisfactory arrangement can be made to sell the surplus at an economic price to the utility company.

     

  • Stand Alone systems
  • The sizing of stand alone systems is however more critical; too large and the system is unnecessarily costly and perhaps uneconomical; too small and the user will suffer power outages.

    Off grid systems must therefore be capable of accommodating peak loads and in the case of solar and wind power, prolonged periods when no external energy input is available. This requires batteries with sufficient storage capacity to cover the lean periods and capable of delivering sufficient power, in conjunction with the generator, to supply peak energy demands. Any surplus capacity is however wasteful.

 

Load Pattern

The key information which determines the size of all system components is the peak and average power which they must handle and this is determined by the user's load pattern. In the section on Energy Demand, there is a table listing the Energy Consumption of various household appliances. Fortunately these are not all used simultaneously, otherwise the peak energy demand would be enormous. On the same page is a graph showing an actual domestic Demand Pattern for a typical single family dwelling which shows how the demand varies during the day.

The system must be sized to be able to deliver the peak power when needed and should have the capacity to deliver the average power on a continuous basis.

 

Inverter

The inverter is the component which delivers the AC power to the user's electrical equipment. It's peak and average continuous output power should therefore match the expected user demand.

Inverters must normally be dimensioned for maximum load taking into account the inrush current surges due to electric motors which could be up to 4 times the rated motor current. Unfortunately efficiency drops off at low power levels so inverters specified for high inrush current applications will be less efficient in normal use.

In addition it is common practice to allow for for potential growth in the use of the system and for occasional overloads by specifying an extra 25% to 30% in the output power rating of the inverter.

No electrical equipment is 100% efficient and the typical losses in the inverter amount to around 10%. Thus to deliver the required output, the DC input power to the inverter should be 10% more than its AC output power.

 

Rectifier, Voltage Regulator and DC-DC Converter

The inverter is supplied by the voltage regulator and DC-DC converter which must provide the calculated input power for the inverter. The battery voltage should be as high as practicable to reduce the step up ratio of the DC-DC converter since the conversion efficiency is lower at high step up ratios.

Losses in these components could also be up to 10% which means that the generator should provide sufficient power to compensate for these losses.

 

Generator

Working back from the user demand, we can find out how much power the generator must provide. The actual size of the generator will depend on the intensity of the energy source and the efficiency of the chosen energy conversion system.

For small scale stand alone generating systems, such as those using solar panels, wind turbines or micro-hydro turbines, it is customary to specify the output power of the generator as sufficient to satisfy the average rather than the peak demand, including the BOS losses, leaving it to the battery to supply peaks. This is because the capital cost of battery power is usually less expensive than the equivalent generator power.

The battery will be charged during periods when the supply exceeds the demand. Once it is fully charged the surplus energy is diverted into a "dump load", such as the domestic heating installation.

Load diversion prevents the turbine from overspeeding.

 

Battery

The power handling capacity of the battery must be sufficient to satisfy the peak demand plus a safety factor of about 30%

The energy storage capacity must be sufficient to maintain the electricity supply when local power (sun ,wind or water) might not be available. The period without local power top up could be as long as two or three days and, unless there is some form of load shedding, the battery should be able to supply the full required load for that period. Again a safety factor of at least 30% additional capacity would be prudent however three to six days storage capacity is not unusual.

Note that there is a round trip energy loss inherent in the charging and discharging process of the battery which should also be allowed for. The ratio between the energy removed from the battery during discharge and the energy supplied to charge it is called the coulombic efficiency. It is typically around 90% depending on the cell chemistry.

Note also that there will be an additional energy loss of about 10% in the charger used to charge the battery.

Battery cycle life is increased if the depth of discharge (DOD) is reduced. Battery life can thus be extended by specifying batteries with a greater capacity than needed so that they are subject to shallower daily depth of discharge. See the section on Battery Life

 

See also Generators

 

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