The Design Process

A correctly specified and installed solar powered irrigation solution can provide long and trouble free service and excellent return on investment. 

When assessing feasibility, size and configuration, key factors include the timing of pumping, the volume and reliability of water supply required, water storage capacity, and the potential to integrate solar with other power sources. 

Farms that have relatively continuous and predictable day-time irrigation needs are ideal candidates for solutions that fully replace mains power. As a result, many horticultural growers have already adopted solar. However, in most broad-acre irrigation systems, pumping requirements are seasonal and vary in response to climate. These systems require more complex assessment and design. 

Where pumping is irregular, or not always in day light hours, return on investment and optimal system size should be calculated with reference to external factors such as other electricity demand on farm, ability to export and sell unused electricity and ability to offset night-time mains electricity cost with savings on daytime usage. 

Prior to committing to a major system we recommend that you undertake the following steps: 

  • Commission a general energy assessment of your farm. This should include accurate documentation of the quantity, cost and timing of energy used by your irrigation system; 
  • Address energy efficiency savings first. Poor layouts, pipe diameters, pump size and maintenance are typical energy wastage points; 
  • Check the capacity of your water storage infrastructure and minimise leakage and evaporation; and 
  • Involve your irrigation engineer to clarify priorities and technical requirements (eg. dynamic pumping head, pressure, control systems, etc.). 

Having this information at hand will prepare you to discuss options with suppliers and obtain accurate quotes. It is essential that suppliers have experience with solar irrigation applications. 

A solar PV pumping system should be designed to optimise efficiency and cost. The steps involved in the design process are shown in Fig.42. 

Figure 42: The steps involved in designing a solar pumping system.
 

Some of the calculations involved in the design of a solar pumping system are very complicated and require a highly technical understanding of the complexities in both pump and array design. The design process above aims to explain the key principles of solar pumping design so that an informed decision can be made about a suitable solar pump supplier and the packages they offer. 

Worked examples 

Throughout this section, we have used two worked examples to help explain the design process and illustrate the application of the required steps (see the series of coloured boxes embedded in the text entitled ‘Dairy farm in Tamworth’ and ‘Small farm in Griffith’). These examples describe the solar PV pumping solutions adopted by two NSW-based farming operations: a Tamworth dairy farm and a mixed livestock and horticulture farm in Griffith. 

Example: Dairy farm in Tamworth 

A dairy farm in Tamworth, NSW, uses a solar pumping system to move water from a river into a stock dam to be used as drinking water for 1,200 cattle. Dairy cows have high daily water needs for milk production, and pumping a large amount of water requires a lot of power. The water for these cattle is lifted through a small height, increasing the system’s power requirements by a corresponding amount. The solar pumping system for this farm was designed so that each day, it is able to move the large amount of water the cattle need over the required distance and height. 

Figure 43: The Tamworth dairy farm solar PV pump.
 

 

Example: Small farm in Griffith 

A small farm in Griffith, NSW, uses a solar PV-powered water pumping station to move water from a bore into a storage tank. From here, the water is gravity-fed into troughs for 50 sheep and to drip irrigate a small orange orchard. The pump is designed to move the daily water requirements of the livestock and crops as well as excess supply that is stored in the tank for days when solar radiation is lower. The farm’s total daily pumping requirement is reasonably low and requires only a small amount of pump power. The water storage tank is located high on a hill, enabling water stored here to be gravity-fed. Moving water to the top of this hill requires a significant amount of pump power. The Griffith farm’s solar pumping system is designed to move the required amount of water through the required distance and height each day. 

Figure 44: The Griffith farm solar pump.
 

A design checklist is included in Appendix C: Solar pump design checklist

Step 1: Site assessment 

An accurate site assessment is critical to successful solar pump design. This ensures that system design and installation locations are site-specific so that the system delivers the required pumping outputs, with the least amount of wasted energy. 

A site assessment involves gathering accurate information about your farm's daily water requirements, the site’s historical solar resource, possible water sources and the water delivery point. 

What information do I need to take to a solar pump supplier? 

Information typically required by solar pump suppliers includes: 

Water requirements

  • for what purpose(s) the water will be used (for example, livestock, irrigation); 
  • your daily water requirements; and 
  • monthly/ seasonal variation. 

Locations for the solar pumping system: 

  • the proposed location (geographical coordinates) for your solar pumping system;
  • the proposed location of the solar array; and 
  • potential sources of shading. 

Water sources: 

  • the type of water source (a bore, dam or river); 
  • the recovery rate of the proposed water source; 
  • the depth of the water source; and 
  • the distance from the top of this water source to the ground. 

Water delivery: 

  • the location to which the water will be pumped to (such as a storage tank or dam, directly to troughs, or to drip irrigation); 
  • the vertical lift between the pump and the water delivery point; and 
  • the length of the route between the pump and the water delivery point. 

 

Daily water requirements 

The farm’s daily water requirements form the central design criterion of a solar pumping system. The capability of a solar PV pumping system to deliver these daily water requirements, given site-specific parameters, will determine the success of the system design. These requirements will influence: 

  • the choice of water source; 
  • the delivery point for the pumped water; 
  • whether water storage is needed or not; 
  • which types of water pumps will be suitable for the application; 
  • the required size of the pump; and 
  • the solar array design. 

The intended water application – for example, the daily water needs of livestock or the irrigation requirements of a crop – will determine the overall daily water requirements. Monthly and seasonal variations should be considered in your calculation of daily water requirements, as these will also impact on the pumping system design (Table 7). 

A table provided by the NSW Office of Water is attached in Appendix D: Office of Water – Water Needs for a Rural Property. This document gives estimates of the water needs of various animals at different stages of their development. Appendix B gives a brief explanation of the various units of measurements for water volume. 

 

Table 7: Basic template to fill out average daily water requirements for each month of the year.
 

Example: Dairy farm in Tamworth 

The dairy farm in Tamworth introduced at the start of this section has 1,200 cattle that need to be supplied by the solar PV-powered water pump. There are approximately 900 milk-producing cows and 100 calves, with the remaining 200 head of cattle made up of dry dairy cows and bulls. 

The NSW Office of Water report (Appendix D) assesses the water requirements of these cattle as follows: 

  • milk-producing cows = 22m./head/yr 
  • calves = 8m³/head/year 
  • dry dairy/bulls = 15m³/head/yr 

The water requirements of these animals vary little from season to season, increasing slightly in summer and decreasing marginally in winter. However, increased solar radiation over summer naturally results in a greater capacity to pump water in a solar-powered system, so we can assume the daily water requirements in each month will be the same. 

Table 8: Water requirements of cattle herd, Tamworth dairy farm.
 

Note: For ease of calculations, this daily water requirement will be rounded up to 65m./day. 

 

Example: Small farm in Griffith 

The small farm in Griffith has 0.25 hectares planted with orange trees and 50 head of sheep to be supplied by its solar-powered water pump. The water requirements of the sheep vary little from season to season, but those of the orange trees vary significantly from month to month. The annual water requirement of the farm’s sheep is 1,900L. Multiplying this figure by 50 (the number of sheep) and dividing it by 365 (the number of days in a year) gives a daily water requirement for the sheep of 260L/day. The water requirements of the farm’s 0.25 hectares of orange orchards (with the addition of 260L/day for sheep) are detailed in the following table. 

Table 9 : Average daily water requirements, month by month, Griffith small farm.
 

Note: The monthly water requirements of orange orchards were estimated using tools published in NSW Department of Primary Industries report ‘Managing citrus orchards with less water’, published December 2006. 

Figure 45: Average daily water requirement for each month.
 

Solar resource 

The solar resource available at a site determines the energy output of a solar PV array installed on that site, and the amount of time for which a solar-powered water pump will be able to operate on a typical day. 

To assess the solar resource at a site, you can use historical solar radiation data, usually given as solar radiation levels received by a horizontal surface. There are several reliable sources for this data. 

  • The Bureau of Meteorology (BoM) (www.bom.gov.au) has measured solar data for most of its weather stations around Australia. This is the best source of accurate historical solar data for anyone designing a solar pumping system in Australia. In Appendix E, we explain how to use the BoM website to collect historical solar data. 
  • The NASA Atmospheric Science Data Center web portal (eosweb.larc.nasa.gov/sse) includes a calculator that enables you to predict average solar data for any location worldwide, using latitude and longitude. This website is a useful resource but does not contain measured historical solar data. 
  • The Australian Solar Radiation Data Handbook (ASRDH) is a quality source of measured solar radiation data for cities and towns throughout Australia. The data for a specific location can be purchased from Exemplary Energy (www.exemplary.com.au). 
  • Some online solar-pump sizing tools contain solar data for many locations around Australia. 

When purchasing a solar pumping system package, however, you should not be expected to have site-specific solar data; the supplier should be able to provide the correct historical solar data for the system design. 

Establishing the average daily solar radiation levels for each month will provide you with enough information to enable an informed assessment of the proposed solar PV pumping system (Fig. 38). Average daily solar radiation levels on site can be measured in peak sun hours (PSH) or in kWh/m². These units of measurements are equivalent and hence, will yield the same values. (Note: if these are given in MJ/m², the numbers will need to be divided by 3.6 to give PSH.) 

What effect will the array tilt have on the solar resource data? 

Usually, historical solar radiation levels are provided as the total solar energy for a day, where sunlight is falling on a horizontal (flat) surface; however, solar arrays are not installed horizontally but are tilted, and the angle of tilt changes the actual amount of solar energy that will be received by the array. To calculate the available solar resource more accurately, select the proposed tilt of the array and correct the historical solar data accordingly. The method for doing this is described in Step 2: Selecting the array tilt. 

Water resource 

The water resource available at a site needs to be assessed so that a suitable source can be selected for the solar pumping system. In many cases, the water source will have already been chosen. The solar pumping system must be designed with the limitations of the proposed water source in mind. 

Many types of water source can be considered. The most common are dams, bores or aquifers, and rivers or creeks. Each source has characteristics that need to be considered when planning a pumping system. For example, bore water may be located deep underground and, if so, it will have a higher head and will pump at a lower flow rate. On the upside, bore water has a more stable availability as it is less affected by seasonal changes. 

When selecting a suitable water source, we suggest you consider the following characteristics (outlined in Fig. 46). 

Table 10: Key characteristics of a water source.
 
Figure 46: Key characteristics of a water source.
 

Example: Dairy farm in Tamworth 

The manager of the Tamworth dairy farm plans to use a river to supply the farm’s cattle with water (Fig. 47): 

 
 
Figure 47: Water resource of the dairy farm in Tamworth.
 

 

Example: Small farm in Griffith 

The manager of the small Griffith farm plans to use a bore to supply water its orchard and sheep (Fig. 48): 

 
 
Figure 48: Water resource of the farm in Griffith.
 

 

Water delivery point 

The water in a solar PV water-pumping system can be delivered directly to the application (for example, drip irrigation) or delivered to a form of water storage, such as a dam or water tank. The location of this water delivery point will form the head requirement of the pump. The higher or further away from the source the delivery point, the greater pumping power the system will require to move water from the source to its point of delivery. 

For a system that will have water storage, it is important to consider the capacity of this storage. This is especially the case if the pumping system will be designed to pump more water on sunny days to compensate for reduced water pumping on cloudy ones. 

This concept is shown for a water tank in Fig. 49 but can also apply to water stored in a dam. 

 

Figure 49: The daily pumping rate of this system has been determined for an average solar day (middle). On these days, the water pumped is equal to the water used. On above-average solar days (left), more water is pumped and stored. This helps supply water on below-average solar days (right). The storage tank needs to be sized so that it can store enough water on sunny days to compensate for the water not pumped in on very cloudy days.
 

A common configuration for a solar pumping system is to install a water tank in an elevated location. The stored water can be discharged and gravity-fed as required, which means there will be water available even when the solar PV array is producing less power than is needed, such as on cloudy day or overnight. If the tank is not elevated and is unable to gravity-feed water, the system will need additional power to move water from the tank as required. 

Example: Dairy farm in Tamworth 

The dairy farm in Tamworth is going to pump water to a stock dam. The capacity of the stock dam is approximately 1ML (1,000,000L), which is more than 10 times the daily pumping rate of 65,000L/day. The stock dam is approximately 100m from the river, with a vertical lift of 10m. 

 

Example: Small farm in Griffith 

The small farm in Griffith is going to pump its water to a water tank. The owner would like to have the equivalent of one day’s pumped water stored in the tank to compensate for days with low solar radiation. The daily water requirement in summer (the farm’s highest daily water requirement) is 12,600L/day. Therefore, a 10,000L or a 15,000L tank could be suitable. 

Solar PV pumping system layout: location of the components 

Once the solar resource, the water resource and the water delivery point have been assessed, an initial system layout can be created. When devising a solar pumping system layout, it is important that distances – horizontal and vertical – between all the system parts are minimised. Both electrical cabling and water pipes experience energy losses, so keeping their lengths to a minimum will reduce energy losses in the system and will result in the most efficient solar PV pumping system possible for your site and needs. 

  • Water source: It is usual to select the location of the water source first. Generally, it is the least flexible part of the system. 
  • Water pump: The water pump should be situated so as to minimise pipe lengths and electric cable lengths. If using a surface pump (a pump installed on the ground above the water), the distance between the pump and the water source will need to meet the manufacturer’s operating specifications. 
  • Solar PV array: The solar array should be located to avoid shading and to minimise incidental shading, away from tall vegetation and structures. The array should be located so as to minimise the length of electrical cabling needed to reach the water pump. Whether it is roof-mounted or ground-mounted, the solar PV array must be installed on a suitable surface that can adequately support the array. 
  • Water delivery point: The water delivery point should be selected to minimise the head (vertical height) and water-piping distance, while still fulfilling the water delivery requirements. For example, if a water tank is to be used for gravity-feeding, it will need to be located at a height sufficient to enable it to supply the required water application (Fig.50). 
Figure 50: The water tank is sited on a hill so it is able to gravity-feed all the farm’s water applications. The pump has been sited inside a bore hole, with the solar PV array located nearby in a location with minimal shading.
 

Example: Dairy farm in Tamworth 

The dairy farm in Tamworth will have the following system layout: 

  • Water pump: The water pump will be located in an existing pumphouse on the river bank. The pumphouse has a firm foundation that can support the pump and will protect the pump from weather. The distance between the pump and the river is minimal (just 3m of pipe will be required) and there is a suitable array location nearby. 
  • Solar PV array: The solar array will be located approximately 20m from the water pump on a raised flat. There are some small bushes nearby, which would require trimming once a year, but otherwise there are minimal sources of shading. 

The water source and the delivery point have been established in previous sections. 

 

Example: Small farm in Griffith 

The small farm in Griffith will have the following system layout: 

  • Water pump: The water pump will be located in a new bore that is being installed at a low point on the farm. This is a suitable location, as the bore water level will not be excessively deep at this point and there is a suitable location for the solar PV array nearby. 
  • Solar PV array: The solar array will be located approximately 2m from the bore. This area is mainly grassy and will have sheep grazing on it regularly, which will keep grass levels low. The nearest tree is 20m away on the south side of the array, so it will not shade the array. 

The water source and delivery point have been established in previous sections. 

Step 2: Selecting the array tilt and orientation 

The tilt of the array  will affect the amount of solar radiation it receives throughout the year, and its orientation will affect the amount of solar radiation it receives throughout the day. These, in turn, will affect calculations of the flow rate that will be needed for water pumping. Therefore, the next step in the design of a solar PV pumping system is to determine the array tilt and orientation and calculate the resulting solar radiation levels. The size of the array does not need to be ascertained at this point. 

To select an appropriate array tilt and orientation, you need to consider your seasonal/monthly water requirements. If your farming enterprise has significantly greater pumping needs in summer than in winter, the array should be designed to receive more solar radiation during the summer months than in the winter ones. A fixed tilt equal to the latitude of the site is generally a good choice. For a fixed tilt array, this will result in the greatest possible amount of solar radiation over the year, with more solar radiation received in the summer months than in the winter ones. 

If your farm’s pumping needs are similar year-round, or you have significantly greater pumping needs in winter than in summer, the tilt should be set so that the modules receive a more even spread of solar radiation throughout the year (Fig. 51). If you want a fixed array to receive more winter and less summer radiation, consider a slightly steeper tilt. A manual tilting or tracking array could also be suitable. 

Figure 51: a) A lower tilt will give greater solar power generation in summer as then, the sun is higher in the sky; b) a tilt equal to the latitude of the site will give the greatest annual solar-power generation; c) a steeper tilt to a solar PV array will yield greater power generation in winter, when the sun is lower in the sky.
 

A single-axis tracking system increases the output of a solar array in the mornings and afternoons; a dual-axis tracking system would, additionally, increase the output of the array in winter and in summer. It should be remembered, however, that a tracking system increases the costs and maintenance requirements of the array. It is often more cost-effective to install a larger, static array. To give increased morning and afternoon output, consider an array made up of east, north and west-facing solar PV modules. 

There is more than one method by which to calculate the amount of solar radiation your array is likely to receive once the module tilt has been selected. 

  • The NASA Atmospheric Science Data Center (eosweb.larc.nasa.gov/sse) can predict monthly averages for your location for the following tilts: (i) equal to the latitude of the site, (ii) equal to the latitude of the site plus 15⁰, and (iii) equal to the latitude of the site minus 15⁰. For Tamworth, NSW (latitude 31⁰), it can predict average daily radiation levels for each month for the following tilts: (i) 31⁰, (ii) 46⁰, and (iii) 16⁰. (See Table 13). 
  • The Clean Energy Council (CEC) provides tables for Australian capital cities that show increases or decreases in monthly radiation levels according to the tilt and orientation of a solar PV array (www.solaraccreditation.com.au/installers/compliance-and-standards/accred...). An extract of these tables is shown in Table 14. 
  • The Australian Solar Radiation Data Handbook (ASRDH) contains the monthly and annual radiation levels that would be received by a solar PV array using every possible combination of tilt and orientation, for cities and towns throughout Australia. The data for a specific location can be purchased from Exemplary Energy (www.exemplary.com.au). 

How to use the Clean Energy Council (CEC) tilt and orientation tables 

The CEC provides the percentage increase or decrease for different tilts and orientations for Australian capital cities. Use the table for the city with the latitude closest to the proposed solar pump location. This will give a good estimate of the effect on the amount of solar radiation received that would be introduced by the tilt of the array, although there will be differences thanks to differences in the climates of the two locations. 

Table 11: Monthly daily solar radiation levels on an inclined plane for an array facing north in Sydney (latitude 33⁰): Monthly solar radiation levels on an inclined plane for an array facing north in Sydney (latitude 33⁰).
 

To calculate the corrected average PSH, multiply the initial average daily PSH values on a horizontal surface from Table 13 (in page 58) by the percentage from this table for each month. If the tilt of your proposed array is not shown, use Table 11 to estimate the values. 

Once you have selected the optimal tilt for the proposed solar array using the suggested methods, you’ll have the information you need to complete Table 12. 

Table 12: Average daily solar radiation levels for each month, depending on the array tilt.
 

Example: Dairy farm in Tamworth 

The Tamworth farm (latitude 31⁰) has large and consistent water requirements (65m./day) year-round. To maximise the amount of solar radiation received in winter as well as summer, they’ve chosen a manually tilting array that will be tilted approximately 15⁰ every three months in line with the calculations in Table 2: Monthly and quarterly manual tilt plan. This will give them the average daily solar radiation levels shown in Figure 52. Data on horizontal and tilted radiation was sourced from NASA Atmospheric Science Data Centre. 

Table 13: Average daily solar radiation on horizontal and tilted array surfaces (Source: NASA Atmospheric Science Data Center).
 
Figure 52: Average daily solar radiation levels on a manually tilted array.
 

 

Example: Small farm in Griffith 

The small farm in Griffith (latitude 34.29⁰) has significantly greater water requirements in summer (around 12,600 L/day) than in winter (around 2,800 L/day). The owner would like to install a fixed-tilt array, tilted at an angle of 20⁰, slightly flatter than the location’s latitude. This will result in higher solar radiation levels in summer than in winter (Fig. 54). The radiation data was calculated using the CEC table for Canberra, which has a latitude (35.3⁰) and climate similar to those of Griffith. 

Table 14: Average daily solar radiation levels for a horizontal and a tilted array in Griffith, NSW.
 
 
 

Step 3: Calculate the required flow rates 

As explained in the earlier section on pumping, flow rate is the amount of water that can be pumped within a certain time period, for example, per day, or per hour. For a solar PV-powered pumping system, the required daily and hourly flow rates may need to be adjusted to account for variations in levels of solar radiation that are experienced at the site over the course of the day and the year. 

Generally, a pump is selected based on one flow rate, which is quoted in the product’s specifications. You will need to select just one flow rate from the range of monthly flow rates predicted by the solar pumping system. 

Calculate the required daily flow rate 

The required daily flow rate determines the size of the pump required as well as the size of the solar array that will be needed to power the pump. 

Solar radiation levels can vary substantially from one day to the next. On some days, there will be less solar power generated to pump the required water, while on other days, excess solar power will be produced, and additional water will be pumped. 

Following are two methods by which you can calculate the daily flow requirements your system will need to cater for variable solar radiation levels at the site. 

Option 1: Design the daily flow rate around a day of average solar radiation 

If you choose this option, the performance of your solar pumping system will be based on an average solar day. On some days, the water pumped will be less than your operation’s average daily water requirement; on others, the water pumped will be more than the daily requirement. On average, though, the daily water requirement will be delivered. 

If designing the system around an average solar day, you must accept that the solar resource will vary. Unless you build a system that is sized to compensate for this variability, your worst-case scenario could be a run of multiple days of low solar radiation levels or, in any one year, solar levels that are less than the predicated historical averages. 

This option best suits farmers planning a solar PV-powered pumping system that incorporates some form of water storage, or with pumping requirements that are not highly sensitive to variations in the amount of water pumped from day to day. 

In this option, the daily flow rate would be equal to the daily water requirements, as calculated in the site assessment section on ‘Daily water requirements’ (page 46). 

Option 2: Design the daily flow rate around a day of lower-than-average solar radiation 

If you select this option, the performance of the pumping system will be based on a day of lower-than-average solar radiation. There may be some days when pumping output will be less than average but the system will be designed so that, on these days, the system will pump more water than it would if you’d chosen Option 1. On sunny days, the system will pump more than the average requirement, and this additional water can be stored for later use. This means that, on average, the pumping system’s daily flow rate will be greater than the daily water requirements. 

This is a solar pumping system that is able to compensate for solar variability, and one that makes it possible for multiple days of low solar radiation levels to be compensated by a combination of higher pumping levels and water storage on sunny days. 

This option would be most suitable for a solar PV-powered pumping system that is reasonably sensitive to variations in the amount of water pumped, or for a system that is unable to include a storage facility. 

In this option, daily flow rate would be greater than daily water requirements as calculated in the site assessment section Daily water requirements (page 46). 

Daily flow rates can vary monthly and/or seasonally. 

Table 15: The daily flow rate that is required each month.
 

Example: Dairy farm in Tamworth 

The dairy farm in Tamworth has a large water storage area (the stock dam), which can store extra water pumped on sunny days to compensate for cloudy days when less water is pumped. 

As cattle water needs are not highly sensitive to fluctuations in supply, there’s no need for the daily flow rate of this farm’s pumping system to exceed the daily water requirement. The daily water requirement for this farm remains constant throughout the year at 65m³/day. 

 

Example: Small farm in Griffith 

The small farm in Griffith has citrus trees, so a reliable water supply is important during the summer (fruit-growing season). The orange trees are reasonably tolerant of daily fluctuations of water supply, however, and the owner plans to install a water storage tank that will have the capacity to store the additional water pumped on sunny days so it can be used to provide water on cloudy days when less water is pumped. In this case, the daily flow rate does not need to be more than the daily water requirement. 

Daily water requirements for this farm are not constant year-round; being greater in summer (around 12,600 L/day) than winter (around 2,800 L/day). As the pump will need to be able to operate at the highest flow rate required, the system should be based on the summer daily water requirement of 12,600 L/day. 

 

Estimate the required flow rate per hour and per minute 

Many manufacturers provide pump specifications using measured flow rates (m./hr, L/hr or L/min), so it can be useful to have estimates of the required flow rate per hour and per minute at hand. 

These flow rate figures can be estimated by using average solar energy per day, measured in peak sun hours (PSH). The greater the number of PSHs at a site on an average day, the more solar resource is available at that site per day. This means that for a given total pumping requirement, a pump with a lower flow rate can be selected for that site (if the solar resource is less, a pump with a higher flow rate could be required). 

As average daily PSH changes throughout the year and required daily flow can also vary significantly over that period, it is useful to calculate flow rates separately for each month. 

To calculate the flow rate per hour, use the following equation, remembering that 1m³ = 1,000L. 

 
 

This equation can be used to fill in the following table (Table 16). 

 

Table 16: Estimated flow rate per hour each month.
 

To calculate the flow rate per minute, use the following equation: 

 
 

In general, a solar PV pump will be selected on a single flow-rate value, with different flow rates calculated for each month. There are a few options for arriving at a single flow-rate value:

  1. use the highest flow-rate value, which will mean that the pump should deliver the required daily water quantity in even the worst month for solar radiation, and will pump an excess quantity of water in other months,
  2. use the lowest flow rate, which means that the pump should deliver the required daily quantity of water only in the best month for solar radiation and less in other months, or
  3. use an in-between flow rate, which means that the pump will deliver less than the required quantity of water in the worst months for solar radiation, and more in the sunniest months. 

When choosing among these options, base your selection on the sensitivity of the application to receiving less water in some months. 

Example: Dairy farm in Tamworth 

The flow rate per hour for each month for the dairy farm in Tamworth is as detailed in Table 17. 

Table 17: Flow rate per hour for each month at the Tamworth, NSW dairy farm.
 

Due to the critical need to provide water for stock, the highest flow rate should be selected for the pump that corresponds to the flow rate given for June/July, that is, 14.4 m³/hr. 

 

Example: Small farm in Griffith 

For the small orange grove and sheep farm in Griffith, the flow rate per hour for each month is as detailed in Table 18. 

Table 18: Flow rate for the small farm in Griffith, NSW.
 

In this case the critical needs of both the orchard and the animals also justify selecting the highest flow rate, which in this case is the flow rate in February (1.7 m³/hr). 

Step 4: Calculate total dynamic head 

As explained in the pumping section earlier, the total dynamic head (TDH) represents the total resistance experienced by the water in the journey from water source to delivery (or storage) point. The pump must have the operating capacity to overcome the total dynamic head to be able to move the water to the required destination. The TDH includes the height through which the water needs to be lifted (static head) as well as the friction of the water running through the pipes (dynamic head). 

Static head: This is the total difference in elevation between the pump and the water destination. 

Dynamic head: This refers to friction losses in the pipes on the journey from water source to delivery point. 

The TDH is equal to the sum of the static head and the dynamic head: 

Total dynamic head (TDH) = static head + dynamic head

The design of your solar PV pumping system should include a safety margin for the calculated TDH. It would be appropriate to add 20 to 30 percent to the TDH for this margin. 

Calculating the static head 

The static head is the vertical distance that the water travels on its pumping journey. 

For a submersible pump (Fig. 54a), the static head is the height difference between the pump and the water destination. This is equal to: 

Static head (submersible) = drawdown level + static water level + lift from surface

For a surface pump (Fig. 54b), the static head is the height difference between the top of the water source and the water destination. This is equal to: 

Static head (surface) = suction lift + lift from surface

Figure 54: a) The static head of a submersible pump; b) the static head of a surface pump.
 

Calculating the dynamic head 

Ascertaining the dynamic head of a pumping system involves complex calculations. Pump Industry Australia has produced a textbook on the ins and outs of pipe friction. Examples of dynamic head calculations are included in Appendix F: Calculating dynamic head (page IX) 

Several websites can help you to calculate a pumping system’s TDH: among them is www.pumpworld.com/total-dyanmic-head-calculator.htm. In general, dynamic head friction losses are affected most by pipe diameter; a larger pipe diameter can reduce the dynamic head significantly, especially in a pumping system with high flow rates. Next are the friction losses due to changes in water direction, obstructions or changes in the pipe diameter. Therefore, pipe fittings such as elbows, filters and valves should be used only where necessary to avoid pointless increases in the system’s dynamic head. Friction losses due to the length of the pipe are generally minimal in comparison. 

Most pump manufacturers and suppliers can provide tools with which you can calculate or estimate the dynamic head of your proposed system based on standard piping options. 

Selecting the water pipes 

The diameter and material of the water pipes you choose will affect the dynamic head of the system. Larger-diameter and/or better quality pipes will reduce the dynamic head (Fig. 55); installing piping of a suitable diameter and quality will minimise system losses and could reduce the size of the pump required (although a larger-diameter and/or better-quality pipe will increase the cost of the system). 

Selecting the most appropriate water pipes entails balancing the additional costs incurred against the benefits of increased system efficiency. 

Figure 55: Using a larger-diameter pipe will reduce the dynamic head but increase the system cost.
 

Example: Tamworth dairy farm 

The total dynamic head (TDH) is to be calculated for the dairy farm in Tamworth. Here, water is to be moved from a river to a stock dam. The following information was calculated in earlier examples. 

  • Static water level = 1m 
  • Length of piping required between river and pump = 3m. 
  • Length of piping required between pump and stock dam = 100m 
  • Vertical lift between ground level and stock dam = 10m 
  • Daily flow rate = 65m³/day 
  • Maximum hourly flow rate = 14.4m³/hr (June) 

The static head is equal to the static water level plus the vertical lift, giving 11m. The dynamic head is too complicated to calculate manually, so www.pumpworld.com/total-dynamic-head-calculator.htm was used to calculate the TDH (i.e. static head + dynamic head) for this site. 

For the dynamic head calculation, pipe size must be known. Selecting an appropriate pipe size involves some trial and error. Finding the right size is a balance between apipe wide enough to ensure that the dynamic head is not excessive but not so wide that the pipe price is unreasonable. In this example, the TDH was calculated for a range of pipe sizes and the most appropriate size of pipe was then selected. 

 
 

A 63mm pipe gives a total dynamic head of 13.6m (11m of static head and 2.6m of dynamic head). This appears to be a reasonable dynamic head compared to the static head, so a 63mm pipe will be selected. 

 

Example: Griffith small orange farm 

The total dynamic head (TDH) is to be calculated for the small farm in Griffith. The water is to be moved from a bore to a water tank. The following information was calculated in earlier examples. 

  • Static water level = 15m 
  • Drawdown of pump (height between turned-on pump and total of water level) = 5m 
  • Length of piping between pump and stock dam = 300m 
  • Vertical lift between ground level and stock dam = 40m 
  • Maximum daily flow rate = 12.6m³/day (February) 
  • Maximum hourly flow rate = 1.7m./hr (February) 

The static head is equal to the static water level plus the drawdown plus the vertical lift, giving 60m. The dynamic head is too complicated to calculate manually, so www.pumpworld.com/total-dynamic-head-calculator.htm was used to make these calculations. 

For the dynamic head calculation, the proposed pipe size must be known. Selecting a suitable pipe size requires one that is wide enough to ensure that the dynamic head is not excessive but not so wide that the pipe price is unreasonable. Thus, the TDH was calculated for multiple pipe sizes before the size deemed most appropriate was selected. 

 
 

A 32mm pipe gives a total dynamic head of 64.0m (60m of static head and 4m of dynamic head). This appears to be a reasonable dynamic head compared to the static head. As the pipe in this system is quite long, using a narrower pipe could reduce the cost of the system significantly. Therefore, a 32mm pipe will be selected. 

Step 5: Select the pump/array 

To complete the solar pumping system design, the pump and array need to be selected. The previous sections lay out the principles in refining your requirements. This will give you confidence and a basis on which to compare the offerings of different pump installers/manufacturers. While the manufacturer should be able to provide a suitable pump/array package, it is still useful to be able to estimate head and flow requirements and to understand the general principles of selecting the pump and array size. 

Following these sections is a brief overview of solar-powered pump-sizing software tools supplied by leading pump suppliers. 

Pump selection 

You should select a pump that is capable of the required flow and head. Solar pump manufacturers generally present their pump selection information in one of two ways. 

  1. Pump performance curves: The pump performance curves below list flow rate on the vertical axis, power on the horizontal axis; each line represents a different total dynamic head (TDH). For a particular TDH, the possible flow rates of each pump can be determined. The power required to achieve each flow/head combination is found on the horizontal axis (Fig. 56). 
Figure 56: Pump performance curves for two pumps; a) SQF 1.2-2 and b) SQF 5A-6..
 

 

Example 

Using Fig. 56 a and Fig. 56 b, which pump is most suitable for a system with the following parameters? 

a) Total dynamic head = 60m , required flow rate = 1 m3/hr 

b) Total dynamic head = 30m, required flow rate = 2 m3/hr 

c) Total dynamic head = 10m , required flow rate = 8 m3/hr 

Answers 

a) SQF 1.2-2 (Figure 56 a): This is the only pump that can reach a head of 60m. The power required for this head and flow combination would be ~300W. 

b) SQF 5A-6 (Figure 56 b): Even though both pumps can reach a head of 30m, only the SQD 5A-6 can pump the required flow rate at this head. The power required for this head-and-flow combination would be ~470W. 

c) SQF 5A-6 (Figure 56 c): This is the only pump that can reach a flow rate of 8m3/hr. The power required for this head-and-flow combination would be ~1,000W. 

 

  1. Pump/array performance tables (Table 19): These tables usually have solar energy listed at the top, head on the left and the flow rate for each combination. For a solar array of a particular size, the possible head heights of each pump can be determined. The maximum flow rate for each combination can then be ascertained. The pink, green and purple boxes show three different sizes of pump. 
Table 19: Pump/array performance table for a candidate Submersible Pump, based on a 6.5kW/hr average performance.
 

It is important to note the assumptions many manufacturers make when providing sizing information. For example, Mono solar pump brochures show their pumps’ performance at a 6.5kW/hr performance level. This correlates to a day with 6.5 PSH of solar radiation, which would be a clear summer day. It will not deliver the listed performance on an average winter day. Mono’s solar pump sizing program allows you to input more detail, however, and can be used to find a solar pump that will also deliver the required flow rate on an average winter day. 

The type of solar-powered pump that is suitable depends on the application. Various types of pumps are explained in Sections 6.3 and 6.4, along with their suitability for different applications (as summarised in Table 20): 

Table 20: Summary of the suitability of different pump technologies and installations for various applications.
 

Selecting the solar array 

Matching a solar array to pumping needs involves complex calculations. It is not as simple as selecting an array with the power requirements to supply your pump, for the following reasons: 

  • The power rating of solar modules is given according to standard test conditions (see ‘Datasheet values: ratings of a PV module ’, page 13) This is generally higher than what will be delivered in real world conditions. A good rule of thumb is that modules will operate at about 75 percent of their rated power, at most, due to losses; 
  • The variability of solar radiation levels means that the power output of the array will vary throughout the day. This is particularly the case when you’re running a centrifugal pump, which can have significantly different efficiencies when operating at different power levels; 
  • The array must be configured to match not only the power requirements but the voltage and current requirements of the motor pump; 
  • The power requirements of the pumps may or may not include the efficiency of the motors. This efficiency is usually around 80 percent; and 
  • If you are considering a grid connected solution or generating power for other on farm uses, you need to factor in total farm electricity needs, pattern of use, and optimisation of tarrifs. 

It is advisable to work with a solar provider who has extensive experience with pumping systems, working with irrigation engineers and with the optimisation of farm electricity supply. 

Various types of modules are available, and are addressed in more detail in ‘Composition and types of solar modules’ (page 15). If given a choice of modules, remember that ‘higher efficiency’ relates to the size of the module, not to its power output. Paying extra for high-efficiency modules may not be worth your while, especially if installation space is not restricted. 

When selecting solar modules or a package, it is important to check that the modules you’re considering are approved by the Clean Energy Council and meet all relevant standards. There are two lists of approved modules: those approved for use on buildings; and those approved for use for ground-mounted systems. Most solar pumps use ground-mounted systems. For detailed information about this form of approved module, go to www.solaraccreditation.com.au/products/modules/modules-for-ground-mounted-systems.html. 

Example: Solar pump sizing software 

In Australia, the leading solar-pump companies offer sizing software that can be used to ascertain the most appropriate solar pump package for your location and applications, based on information you provide. We recommend that you search for available software packages. A query for the terms ‘solar pump sizing software’ or ‘solar pump calculator’ should produce useful results. 

Mono Pumps sizing software: CASS 

The Mono Pumps sizing software is known as computer-aided solar simulation, or CASS, downloadable from www.solarcass.com. It contains the company’s four main solar pumping products, any of which might be used for farm-scale pumping. The products are the Sun-Sub (a bore pump), Sun-Ray SRX Floating, Sun-Ray SRX Surface and SB3 (for small bores). You select the Australian city closest to your farm’s location and inputs the site parameters. The software then calculates average daily and hourly flows for each month of the year and suggests appropriate Mono products to suit your requirements. 

Example in CASS: Tamworth dairy farm 

The requirements for the dairy farm in Tamworth have been inputted into the CASS software for the Sun-Ray SRX Surface Pump (Fig. 57). 

Figure 57: Inputs into CASS.
 

Notes 

  • A stationary array type was selected even though the farm owner plans to use a manually tilting array. This is because the other options listed by CASS are a one-axis and a two-axis tracking array, either of which would have a significantly higher output. 
  • For this farm, ‘Flow Req Type’ must be selected as ‘worst’ as the pump it plans to use is required to move the required flow even in the months of lowest solar radiation, June and July. 

After you select ‘Calculate’, the software determines the array size and pump type that would be needed to deliver the required daily flow to your applications. For this example, a 1,600W array with the listed MPPT, pump and motor will supply the minimum of 65m./day on an average day each month of the year (Fig. 58). 

 

Figure 58: Output of CASS.
 

The software can also calculate an hourly flow rate for an average day of each month (Fig. 59). 

Figure 59: Hourly flow rate for a) June; and b) January.
 

Grundfos sizing software: WebCAPS

The Grundfos sizing software is called WebCAPS and can be found at http://net.grundfos.com/Appl/WebCAPS. It works only for the company’s bore-pump products, the SQF range, although the site gives you the option of selecting surface pumps. Simply choose the Australian city that is closest to your location and input the requested site parameters; the software then suggests the most appropriate Grundfos pump for your needs along with some alternative options. 

Example: Griffith small orange farm 

The requirements for the small farm in Griffith have been inputted into WebCAPS (Fig. 60). 

Figure 60: Inputs into WebCAPS.
 

Notes 

  • Convert units of measurement into Australian (metric) data using the settings button at the top right-hand side of the page. Australia’s power supply frequency is 50Hz; it uses SI units rather than US units. 
  • Pipe system friction losses may need to be inputted manually. The total dynamic head calculator given in Section 6.2.2 can be used to check these. 
  • ‘Peak month’ is the month in which you anticipate having the highest water usage. 
  • ‘Solar modules’ must be selected (with, the numbers representing the modules’ wattage and the letters the type of module). In this instance, 250W polycrystalline modules were selected, as these would be suitable for most farm pump systems. Lower wattage modules were chosen, as these would be well suited to a small pumping system. 
  • ‘Control box options’ fall into two main categories: CU units, for regular solar pumps, and IO units for solar pumps with additional generator connections. 
  • An inverter needs to be selected only for a large system. 
  • The ‘pump material’ and ‘outlet’ boxes can be left blank. 

After the user selects the ‘Start Sizing’ button (at the bottom right-hand corner of the web page), Grundfos recommendations are supplied (Fig. 61). 

 

Figure 61: Grundfos recommendations.
 

The text in the box on the right-hand side of this figure can be summarised as follows: 

  • Pump: SQF 2.5-2N 
  • Array: 4 x SW 250 polycrystalline modules 
  • Controller: CU 200 

More information about the output of this configuration can be found by clicking ‘Alternatives’. This provides some alternative options and indicates the estimated pumping performance of the configuration over four months: January, April, July and October (Fig. 62). 

Figure 62: Alternative options given by the Grundfos sizing tool.
 

The complexity of sizing a solar array is highlighted by comparing the array size recommended by the Grundfos sizing tool with the power requirements gleaned from the pump curve. Grundfos’s sizing tool recommends a 1,000W array (4 x 250W) coupled with the pump SQF 2.5-2N. The pump curve for this pump is shown in Fig. 63. Using a flow rate of 1.7m./hr and a head of 64m with the pump curve, you get an estimated pump power requirement of approximately 600W; however, Grundfos’s sizing tool takes into account the fact that available solar power varies throughout the day and includes system losses to arrive at an array size of 1,000W. 

 

Figure 63: Grundfos pump curve for SQF 2.5-2N.
 

 

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