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Installing a Captive Solar Power Plant

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There are seven steps involved in designing a successful captive solar PV installation

  • Scoping of the project
  • Calculating the amount of solar energy available
  • Surveying the site
  • Calculating the amount of energy needed
  • Sizing the solar system
  • Component selection and costing
  • Detailed design

Step 1 – Scoping of the project

As with any project, you need to know what you want to achieve. This basically involves detailing what you want from the captive PV installation, once installed. Do you want it to completely provide your day time electricity usage? Or do you want it to support a part of your usage? To start with, the scope of the project can be simple and later as we progress we can flesh it out to suit the requirements.

Defining the scope is in fact the most important step because once the basic scope is wrong, we might not be able to get the system do, what we exactly want it to do.

Step2 - Calculating the amount of solar energy available

Solar insolation is the amount of electromagnetic energy (solar radiation) incident on the surface of the earth. Basically that means how much sunlight is shining down on the area under consideration.

The values are generally expressed in kWh/m2/day. This is the amount of solar energy that strikes a square metre of the earth's surface in a single day. Of course this value is averaged to account for differences in the days' length. There are several units that are used throughout the world.

By knowing the insolation levels of a particular region one can determine the number of PV modules that are required. An area with poor insolation levels will need a larger number of PV modules than an area with high insolation levels. Once the region’s insolation level is known, one can more accurately calculate collector size and energy output.

The typical thumbrule that is used for the amount of electricity that solar PV can produce is as follows: On average, 1 W of solar PV, at current crystalline silicon panel efficiencies, can produce about 4 Wh of electricity per day. This is however only an average estimate and based on the location, this could be a bit lower or higher than the average.

Step 3 – Surveying the site

A site survey basically consists of a brief interview with the developer to get a feel for their electricity needs and a physical inspection of the proposed array site to see if it is suitable for solar. When a qualified photovoltaic design professional visits a potential solar site, he or she has many things to watch out for.

Primarily, they will be checking the roof's orientation (azimuth) and solar access. Orientation refers to the direction the roof faces - directly south is ideal, with some leeway to the Southwest or Southeast.

Solar access quantifies the percentage of time when the proposed array location will be receiving the full unshaded power of the sun during different days of the year. A shady roof might disqualify a site from receiving incentive money from the state, and is not a responsible choice for solar anyway. There are ways to get around shade issues - either by looking at alternate sites, trimming or removing trees, or by using micro-inverters in the system design rather than one large central inverter.

Step 4 - Calculating the amount of energy needed

The next big task for any photovoltaic system designer is to determine the system load. This load estimate is one of the key factors in the design and cost of the stand-alone PV system.

A simple way to determine the approximate ceiling for the solar PV system capacity for all electricity needs is as follows:

1.      Find out your total monthly electricity consumption. Let’s say it is 100000 kWh

2.      Divide it by 30 to get an approximate daily consumption. In the example, it is about 3300 kWh.

3.      Using the thumbrule that 1 W of solar PV can approximately produce 4 Wh of electricity per day, you can determine the approximate maximum solar PV capacity you will require to power all your systems using solar PV. In this case, if the total daily consumption of electricity is 3300 kWh, you will require a maximum of 3300/4 = 825 kW.

4.      It is however very unlikely that you would require such a high capacity for solar PV as you will need solar PV primarily as a backup power source, perhaps as a replacement for diesel based power generation.

 

Ceiling for the solar PV required for complete diesel replacement

5.      In most cases, you will be using solar only as a backup power source to replace diesel based power production.

6.      One simple way to determine the amount of solar PV for this purpose is to determine the total amount of electricity you produce using diesel every month. In the example provided, out of the 100000 kWh of total electricity you consume every month, let’s say 10% or 10000 kWh is generated from diesel gensets. This provides you the ceiling for the solar PV system capacity for complete diesel replacement. In this case, it is 82.5 kW.

7.      As a thumbrule, one liter of diesel produces 4 kWh, so you can also compute the above with the data for the amount of diesel used every month.

While estimating the load, the designer should consider energy conserving substitutes for items that are used often. Identifying large and/or variable loads and determining if they can be eliminated or changed to operate from another power source will save cost.

Step 5 – Sizing the system

From the results obtained in step 2 and step 4, we can determine the size of the solar system that will be needed to power the site. The necessary systems involved in the setting up of captive power plants are:

1)      Array(collection of solar PV modules)

2)      Charge controllers

3)      Batteries

4)      Inverters

5)      Mounting systems

Note: The exact procedure for sizing of a solar system has to begin with calculating the amphere hour (Ah) of each load under consideration. Without knowing this it is impossible to size the PV system.

PV array sizing – Array sizing is determined by taking into account the daily energy requirement (in Kilowatt hours) and average daily peak sunshine hours in the design month.

No part of a PV array can be shaded. The shading of small portions of a PV module may greatly reduce output from the entire array. PV modules connected in series must carry the same current. If some of the PV cells are shaded, they cannot produce current and will become reverse biased. This means the shaded cells will dissipate power as heat, and over a period of time failure will occur.  However, since it is impossible to prevent occasional shading, the use of by pass diodes around series - connected modules is recommended

Hybrid Indicator

At this point, the basic PV system configuration and size have been determined. Before proceeding to specify components for the system, a simple test is recommended to see if the application might be a candidate for a hybrid system.

Two main indicators work together to alert the designer to a possible hybrid application; the size of the load, and the seasonal insolation variability at the site.  The larger the load the more likely a hybrid PV-generator system will be a good economic choice. Likewise, in cloudy climates you need a larger system to meet the load demand; thus having a higher array/load ratio. Plotting the load versus the array/load ratio gives an indication of whether a hybrid system should be considered. There may be other reasons to consider a hybrid system: for example, systems with high availability requirements or applications where the load energy is being provided by an existing generator.  A word of caution--the controls required for a hybrid system are more complex because the interaction between engine generator, PV array, and battery must be regulated. Obtaining advice from an experienced designer is recommended if you decide to install a hybrid system. 

Controllers - Charge controllers are included in most photovoltaic systems to protect the batteries from overcharge or excessive discharge. Overcharging can boil the electrolyte from the battery and cause failure. Allowing the battery to be discharged too much will cause premature battery failure and possible damage to the load. The controller is a critical component in your PV system. Thousands of rupees of damage may occur if it does not function properly. In addition, all controllers cause some losses (tare loss) in the system. One minus these losses, expressed as a percentage, is the controller efficiency.  The cost of the controller increases rapidly as the current requirement increases. Controllers for 12-volt and 24-volt systems with currents up to 30 amperes are available at a reasonable cost. Controllers with 30- 100 amperes are available but 2-5 times more expensive. Controllers that will switch currents over 100 amperes are usually custom designed for the application. One way to work with currents over 100 amperes is to connect controllers in parallel. It is often less expensive to use five 20- ampere rated controllers in parallel than one 100-ampere unit. 

The controller must be installed in a weather resistant junction box and can be located with other components such as diodes, fuses, and switches. Excessive heat will shorten controller lifetime so the junction box should be installed in a shaded area and venting provided if possible. Controllers should not be mounted in the same enclosure with batteries. The batteries produce a corrosive environment that may cause failure of electronic components. 

Battery sizing - To determine the size of the battery storage required for a stand-alone PV system, it is required to make a number of decisions. Before making these choices, one should study and understand battery parameters and the concept of system availability. First, you must choose the amount of back-up energy you want to store for your application. This is usually expressed as a number of no sun days, in other words, for how many cloudy days must your system operate using energy stored in batteries. There is no “right answer” to this question. It depends on the application, the type of battery, and the system availability desired.

Inverters - Power conditioning units, commonly called inverters, are necessary in any stand-alone PV system with ac loads. The choice of inverter will be a key factor in setting the dc operating voltage of your system.

When specifying an inverter, it is necessary to consider requirements of both the dc input and the ac output.   The choice of inverter will affect the performance, reliability, and cost of your PV system. Usually, it is the third most expensive component after the array and battery. 

The selection of the inverter input voltage is an important decision because it often dictates the system dc voltage.

An inverter should be installed in a controlled environment because high temperatures and excessive dust will reduce lifetime and may cause failure. The inverter should not be installed in the same enclosure with the batteries because the corrosive gassing of the batteries can damage the electronics and the switching in the inverter might cause an explosion. However, the inverter should be installed near the batteries to keep resistive losses in the wires to a minimum. 

Mounting structures- Ground mounting of PV arrays is recommended for stand-alone systems. Regardless of whether you buy or build the mounting structure make sure it is anchored and the modules are restrained. Many module manufacturers and distributors sell mounting hardware specifically designed for their modules. This hardware is intended for multiple applications and different mounting techniques and considerations like wind loading have been included in the design. Using this mounting hardware is the simplest and often the most cost effective. Customized array mounting structures can be expensive.   

Others- It is important to select wire, connectors, and protection components such as switches and fuses that will last for twenty years or more. To obtain this long life, they must be sized correctly, rated for the application, and installed carefully. Connections are particularly prone to failure unless they are made carefully and correctly. 

Step 6 – Component selection and costing

Once the various components have been sized, the next important step is the selection and costing of the components.

There are many players in the market vying to establish their products. At this juncture, the system developer has to select components by taking into account factors like technical specifications, reliability, and lifetime of the components in addition to the cost.

Investment for the solar modules is for a period of 25 years, so selecting a high efficient solar panel is of prime importance. The manufactures of the batteries claim a lifetime of about 7 years, whereas inverters guarantee at most 2 years. As can be seen from these numbers, selection becomes a crucial part of the captive solar PV installation.

Step 7 – Detailed design 

Now that the major components have been sized and selected, it is time to consider how to interconnect everything as a working system.

The detailed design is the more actionable form of the captive solar PV installation. The system developer accumulates all the data collected from the previous 6 steps and prepares a layout of the installation on paper. The developer removes obvious engineering fallacies and prepares a corrected version of the layout on paper.

The confirmed design will have all the necessary data like the average consumption per day(kWh), the insolation levels at the area under consideration(in hours) , the optimal plant size, the area required for the same, the number of panels required to be installed in that area, the number of charge controllers, batteries, inverters required for the determined plant size, the cost of all the components and many more intricate details like the viability of installing tracking systems etc.

Stand-alone PV systems will be reliable power producers for more than two decades if properly sized for the application, engineered well, and installed carefully. PV arrays for stand-alone systems are installed in many unique and innovative ways. However, there are common issues involved in any installation, whether the array is fixed or tracking, mounted at ground level, or on a pole or building. 

Preventive Maintenance

The integral part of any completed installation is the periodic checks that are recommended for any stand-alone PV system so that little problems can be found and corrected before they affect system operation. The system should be checked soon after installation when it is presumably operating well.

 

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