CCAT solar irrigation for food forest/Literature review

Literature review[edit | edit source]

This is a review of some available literature pertinent to solar irrigation for food forest project.

• Food forest aspects

Food forests are a means of cooperating with nature to address hunger and nutrition issues in both rural and urban areas.^[1] Even small gardens can incorporate forest aspects that nurture both the environment and the land stewards. Landscapes can be designed and planted in seven layers ultimately providing multiple functions. Taller trees can provide shade and mulch along with fruits or medicinal barks. Lower trees and shrubs can be planted in conjunction with other layers to fixate nitrogen in the soil and provide habitat for wildlife. Herbs and ground cover can also provide these functions as well as attracting pollinators. Root and vine layers can provide tubers to harvest or aesthetic pleasure for those who visit the space. When laying out the design for the food forest, the needs and purposes of end-users should be considered as well as the interrelationships between the plants as some form better companions and help each other thrive with others.^[2] Hardiness zones should also be considered when choosing the plants that will interact in this space.

Intercropping has been proven to increase yields and be more efficient on a land equivalent ratio.^[3] Despite the small space set aside for this project, the potentials for sustainable sustenance harvest and learning from this project is immense.

• Solar basics

Photovoltaics have a history of experimentation and research that dates back to 1839 by a French physicist. Alexandre Edmond Becquerel discovered that conductance will increase with illumination while they were experimenting with metal electrodes and electrolytes. The first germanium solar cells were made in 1951 and Bell's Laboratories published results of a solar cell operating at 4.5% efficiency. The efficiency of solar cells grew to 20% by 1985.^[4]

Solar panels generate electricity by utilizing solar cells. Solar cells can convert sunlight into direct current electricity by using the photon energy of the incident light from sunlight using the photovoltaic effect of a solar cell. Solar cells generally have the same 5 components including; an anti-reflective coating which is typically placed over the bare silicon, front contacts to collect the current the cell generates, there is an emitter that absorbs the incoming photons and transports the energy to the excited state of charge, there is emitter at the base region which is joined at a junction with emitter region, lastly, there is a rear contact.^[4]

With solar arrays/panels there will be energy losses in the system which is often caused by shade and shadows on the panels. Some times and days are considered unfavorable, which is when the sun is at its lowest point in the sky. The most unfavorable day is different depending on which hemisphere the array is located in. The winter solstice, December 21st for the northern hemisphere and June 21st for the southern hemisphere, is considered the most inefficient day when it some to solar arrays. If the panels are angled incorrectly then there is the possibility that the best sunlight will pass over the top of the panel generating an inefficiency.^[5]

There is a growing demand for electricity to be supplied by sources that are not based on fossil fuels, and solar is helping with that need. There were an estimated 272,000 solar PV (photovoltaic) power generating systems a variety of sizes connected to the grid as of 2013. Roughly 4% of those were added in 2011, approximately 62,500 individual systems.^[6]

• Irrigation specifics

Irrigation basics:

Irrigation is essentially a man-made method for transporting water from a source location to a piece of land containing crops in need of water. Irrigation systems can come in a multitude of different forms but almost always use a pump to generate enough pressure to transport the water across the land to the crops. Systems have also been known to use gravity to assist water flow. Irrigation systems require energy to function and in this case, we plan to use solar energy and gravity to power our system. Irrigation systems can run underground, above ground, or both and are usually built to minimize both energy and water consumption.^[7][8]

Irrigation concerns:

To be useful during the drier summer months, the system must be as efficient as possible and conserve as much water as possible as we will be working with a limited water supply (water is from the CCAT rainwater catchment system ). We must maximize crop yields while minimizing unnecessary water usage.^[9] The system could be susceptible to vandalism and solar damage. The system must also distribute water at a controlled rate to avoid drenching plant leaves (which can cause plant sunburns), and roots (which can lead to wilting). The water pressure must also be strong enough to reach the plants but not so strong that the water streams harm the plants. The system will have to factor in the pressure generated by the gravity pulling the water down from the catchment system to the food forest. The water will also have to be distributed according to how many plots the food forest has, so we will need to figure out how to get the water to flow evenly throughout each plot and multiple tubes simultaneously without the water favoring one tube over the other and using only one pump. It is integral that the water in the system is separated from the electrical parts of the system to avoid frying them. If any fertilizer is used or chemicals on the forest then the runoff must be contained to avoid pollution.

Our irrigation method of choice:

We believe that the type of irrigation best suited to our project is the drip irrigation system. Drip irrigation is a system that utilizes gravity and a slower form of water distribution to slowly drip water from the system to the plant's roots. It is considered one of the most efficient methods of irrigation.^[10] This system uses a collection of pipes, tubing, and emitters to effectively distribute water from the initial water source.^[11] This type of irrigation is extremely effective at saving water which can make it ideal for irrigation during long periods of drought. The drip technique limits evaporation by placing the water directly into the plant root area. It directly targets the plants. It has the potential to be extremely effective in a scenario like the one present in our project.^[9] That said, the tubes can be susceptible to sun damage and degradation as a result. Also clogging from the soil is common. The tubes will have to be above ground too which could impact the aesthetic if not desired.^[12]

• Water pump specifics

Pump Basics:

The pump is used to create water pressure for the flow of water through the irrigation system. Flow rate is measured in gallons per minute (GPM)^[13]. The pump converts electrical power to build pressure in the system. This pressure is measured in PSI or Head feet. Head feet are used because manufacturers don't know what the pump is intended to pump. Therefore the PSI will vary based on the liquid's properties being pumped. Selecting the right pump is important to obtain maximum efficiency. Using the most efficient pump will ensure the least amount of power is used and the maximum life will be had of the pump. Finding the amount of energy necessary for the pump can be by:

The hydraulic energy required (kWh/day)
= volume required (m³/day) x head (m) x water density x gravity / (3.6 x 106)
= 0.002725 x volume (m³/day) x head (m) [[14]]

A centrifugal pump will be best suited for the planned drip system as we will be pumping it up hill for gravity to feed the irrigation system. The vertical lift is ~16 feet to the top tank. The drip irrigation will be operating at ~35 PSI^[15]. This PSI is based on water pressure from the gravity fed tank.

Concerns with pumping

When using a pump some of the concerns are focused on efficiency and ability. This requires knowing the output needed for the project.^[16] Then selecting a pump that is right based on the Pump Chart from the manufacturer.^[16] Selecting a pump that is either too large or too small can result in shorter life expectancy and inefficient operating. Another concern is the maintenance of the pump. Making sure the pump is properly maintained to the manufacturer's specs is important to ensure maximum efficiency and life of your pump. Priming the pump is required on centrifugal pumps before startup. failure to do so will result in damage to the pump and the system failing to work.^[17] If the pump operates off of DC it can be hooked directly to our solar panel. However, if it requires AC the power would have to run from the solar panel through an inverter then to the pump. To keep the system as simple and inexpensive for budget as possible a pump operating off of our solar panel's power is preferred. To prevent having to use a battery or multiple panels. For this, we need a pump that operates on 24volts or less and a max of 135 watts (if the current solar panel works). This will help to keep the project on budget as well as keeping the system as user-friendly as possible. Having to do this may require moving the water tank lower on the hill as the pumps may not have as head lift potential.

References[edit | edit source]