Photovoltaic Energy Generation Water Footprint

By guest blog poster Matt Seitzler, PE, The Davis Energy Group

Drought and Water Usage

As a result of last year’s record low rainfall, at the beginning of this year California Governor Jerry Brown declared a state of emergency asking all of California to conserve water during what would end up being an unprecedented three-year drought. Today California faces this drought where nearly 58% of the state is classified by the U.S. Drought Monitor1 as being in an “exceptional level” of drought;  it’s highest level of drought intensity classification. This heightened state of awareness regarding water has brought the issue of water usage across all sectors of society into focus. One area of particular interest not often discussed, is the issue of water usage in the production of electricity. As the US population grows the stress on freshwater resources is now becoming evident especially in areas like the Southeast of the US, where in some cases water usage via the generation of electricity is greater than that used by residences within certain areas2. In fact in the report Burning our Rivers: The Water Footprint of Electricity, the authors calculate that, for an average US household, the amount of

water consumed for electricity generation to meet household loads—using the current portfolio of generation technologies—is five times that of the average water consumption.

Water Footprint for Electricity

All types of electric power generation use water in some form, but not surprisingly this amount is not the same across generation technologies.  As a part of the Burning Our Rivers report, the authors analyzed the water footprint for traditional electricity generation sources (coal, nuclear, and natural gas) as well as emerging resources like renewable energy technologies (solar thermal, geothermal, photovoltaics (PV)). Comparison of water usage is based upon the determination of the water footprint of each

Figure 1. Lifecycle Water Use for Electricity Generation by Fuel Type (Gallons/MWh)
Figure 1. Lifecycle Water Use for Electricity Generation by Fuel Type (Gallons/MWh)

technology in gallons per mWh. Water footprint is classified in terms of the sum of the amount of water used, or withdrawn, and the amount of water wasted, or consumed; usually as a result of evaporation of stored water.  One item to note is that while the water footprint is useful in comparing generation technologies, another important aspect not captured in the volumetric water footprint are the thermal effects of certain power generation technologies on the surrounding environments. As water is often used to cool facilities and then it is ejected into the environment, this practice has been shown to adversely affect nearby ecosystems causing algae and loss of animal species.2

Figure 1. Lifecycle Water Use of Electricity (Gallons/MWh)

Water and PV

The Burning our Rivers report analyzes water use during the upstream or manufacturing phase and during the on-site power generation phase of each of the technologies included. In the chart above, findings from the report show that PV is the second overall least users of water on a gallons per MWh basis. Additionally, the production of PV modules and equipment was found to require a minimal amount of water withdrawn and nearly no water consumed during the manufacturing phase. In the on-site power generation stage of the analysis, PV with its low water use requirements during operation and maintenance enabled it to be the second least user of water in that category as well.

Commonly, PV facilities require small amounts of water in their maintenance typically due to the cleaning of module surfaces resulting from soiling from the environment. Previously done by hand, with the introduction of robotic cleaning devices, the use of water can be more precisely controlled while even some manufacturers are reporting positive cleaning results using robotic cleaning devices without the use of water at all.

So, while the use of PV for power generation might require the use of more land and possible animal habitat, it would not be at the cost of additional water loads and the introduction of thermal load into the environment. With the introduction of new energy storage technologies coupled with a beneficial water footprint, PV is poised to be an even more sustainable way to meet our electric power generation needs.

 References

[1] US Drought monitor website: http://droughtmonitor.unl.edu/Home/StateDroughtMonitor.aspx?CA

[2] Burning Our Rivers: The Water Footprint of Electricity, by W. Wilson, T. Leipzig, and B. Griffiths-Sattenspiel, www.rivernetwork.org

Determining Pile Depth for Utility Scale Solar Farms

Providing optimized utility scale solar engineering designs almost always involves performing an actual field loading and testing of piles at the site during the design phase. Pile load testing is important because if you can save 1 foot of post embedment across thousands of posts, a substantial labor and material cost savings may be achieved.

Techniques

Empirical techniques usually require some type of validation and testing during construction to be in accordance with many building codes, such as the California Building Code (CBC). Additionally, we recommend actual field load testing as the accuracy of empirical estimations is reduced with shallow pile embedment depths as expected for this project. It is our professional opinion that performing actual load testing for design tends to reduce design conservatism (and thus material costs) and can potentially reduce “changed conditions” claims during construction.

We usually propose driving test piles to provide information for the design phase, recording such information as the total depth driven, time to drive each pile, and depths of abrupt changes in driving resistance and refusal. We have assumed mechanical pile driving equipment (with vibratory driving methods) will be an option for installation of the piles during construction and we typically base our proposed technical services around this method.

Methods

As part of this scope we will perform uplift and lateral load testing of piles a minimum of 24-hours after they are driven. We expect to perform the uplift test first, and then apply a lateral load on the same pile. Since the amount of movement associated with the uplift test is very small, the potential for an adverse effect on the lateral load test should be acceptable.

Vertical pile load testing
Vertical pile load testing

Pile uplift and lateral deflection testing shall be performed by using a 12-foot steel load frame with a 10,000 lb. capacity. A calibrated 30,000 lb. capacity hydraulic hollow-ram load device manufactured by Enerpac will be used for load measuring. A reaction frame equipped with 2 digital indicators accurate to within 10

Lateral pile load testing
Lateral pile load testing

thousandths of an inch will be used for vertical and lateral deflection measurements. The testing will be in general conformance with ASTM D 3689-07, Standard Test Method for Deep Foundations under Static Axial Tensile Load and ASTM D 3966-07, Standard Test Method for Deep Foundations under Lateral Load, as modified by our sub for the small piles being tested. The testing apparatus will be transported to the project site by trailer and will be positioned at the desired test location using a backhoe and/or skid steer loader. Once the load testing is complete, we will remove and dispose of the piles.

Deliverables

Our typical deliverable when optimizing pile embedment depths for a site will be a brief narrative of our testing observations and an Excel spreadsheet containing the test results. Our report will include the following:

  • Project identification and location, including test site locations.
  • Descriptions of test, including pile installation equipment, reaction systems, load and deflection measurement devices, weather, unusual events during pile installation, and other factors which may have an influence on the results.
  • Site plan showing approximate pile test locations.
  • Type and dimensions of test piles, length of test pile during driving, final bottom elevations relative to ground elevation, embedded length of test piles, and tested length of test pile. Driving records (date installed, rate of pile penetration in feet/second for driving, cause and duration of interruptions in pile installation, if any, and notation of any unusual occurrences during installation).
  • Tabulated and graphed measurements of load and deflection for each test, with the test number and embedment depth indicated.

Although there is some effort involved in pile load testing for solar projects, we have witnessed this advance planning resulting in site optimization and expense reduction. If Blue Oak Energy may be of service to assist with pile load testing, our civil and mechanical engineering team is a great resource for developing the test plan, deploying the pile load test crews, and summarizing the results in a meaningful format for your utility scale solar facility.

Transformer Basics for Solar Power Plants

All grid-tied photovoltaic systems include a main power transformer to provide galvanic isolation, step up the voltage and supply power back to the utility grid.  A common transformer size for most medium voltage solar facilities is the 0.75 to 2.5MVA, 15kV class step-up product range.  Medium sized PV systems are required to step up their output voltage to around 12kV in order to interconnect at common North American distribution voltages.  Let’s take a moment to consider the fundamentals of how a transformer works.

Transformers work by transforming voltages from the input to the output due to the physics around electromagnetic fields.  The electrical current running through the primary (input) windings produces a magnetic field through a metallic core of the transformer.  This magnetic field has a certain magnetic flux associated with it.  The magnetic flux flows through the surface area of the transformer core until it reaches the secondary (output) winding.  The magnetic flux induces an electromagnetic force in the secondary windings, which produces a voltage.    The number of turns on the secondary winding is directly proportional to the secondary voltage.  The number of turns in the secondary winding relative to the primary winding determines whether the voltage is stepped up or down.

Transformers usually step voltages up for transmission purposes.  On utility grid distribution and transmission systems, electrical energy will be produced at lower voltages and then stepped up to higher voltages in increments such as 128, 230, 345, 500 and 765 kV.  Higher voltages minimize the losses from the inductance or resistance in the wire during the transmission process.  Transformers are used mostly on AC electrical systems to move electrical energy from the power plants to the substations to individual businesses, houses or loads.    This transmission of energy would require a step down from the higher voltages in the power plant to the lower voltage of 120 V typically found in U.S. residential homes.  This voltage step down is done through the use of a transformer and the ratio of how much the voltage is decreased depends on the number of turns in the transformer.  Let’s look at the theory behind the ideal transformer.

Figure 1 – Transformer Diagram

Figure 1 – The primary voltage is either stepped up or stepped down depending on the number of turns in the windings of both sides.
Figure 1 – The primary voltage is either stepped up or stepped down depending on the number of turns in the windings of both sides.

Variable Space

The change in the magnetic flux is the same on both sides. This yields the following equations.

Faraday

Looking at figure 1, we can tell that: Vs=(3/10)Vp. This Ideal transformer steps the primary voltage down by a factor of 0.3! The number of windings affects how much the transformer will either step up or step down the voltage.  In the example, the primary voltage was reduced by 3/10. This happened because the number of turns on the secondary source was less than the number of turns on the primary source.  This decreases the primary voltage by the ratio between the turns.   This effect could also be used to increase the voltage.  By increasing the number of turns on the secondary source, the primary voltage could also be stepped up.  People with solar systems (120V/240V) that produce more energy than their household consumes have to step up to the medium voltage range (12kV) in order to send energy back to the utility grid.

When choosing a transformer in a PV system, it is important to keep the correct power ratings on both the primary side and the secondary side of the transformer.  Transformers are typically rated in kVA (kilo* volt * amp).  The low side of the transformer should be rated to the output power rating of the inverter.  The high side of the transformer should be rated to the grid at the interconnection voltage specified by the utility company.  The transformer should be able to handle the power requirements on both the low voltage and high voltage sides.  All this information Blue Oak Energy takes into account when designing a PV system.

On utility-scale solar energy facilities, voltages are stepped up from the DC-AC inverter to the utility grid voltage.  A transformer on a solar power facility is primarily used to step-up the voltage to deliver the renewable energy to the utility grid.  However, the transformer has some added benefits in that it provides galvanic isolation between the solar facility and the utility grid.  A transformer is essentially and air gap between two conductor windings.  This air gap provides a safety and separation between the grid and the power source which helps protect the grid from power surges, effects from lightning strikes and faults. These are the basic operative points behind transformers, how they work and why they are important as a primary component on solar electric power plants.  Transformers come in all shapes and sizes with many different features.  There are many types of cooling measures, coolants types, bushings, fuses, cabinet features and other options to choose from.

For more information like this, please visit our website and view Tech Talk #8 at http://www.blueoakenergy.com/tech-talk

Designing Commercial Rooftop Solar Projects for Fire Code Compliance

This is a recent article in the August/September 2014 issue of SolarPro Magazine which features Blue Oak Energy’s take on Commercial Rooftop PV Arrays, Designing for Fire Codes.

Many thanks to the teamwork from our talented engineers Jayme Garcia, PE and Matt Stornetta for  writing this comprehensive article in their limited free time!

Check it out:

Commercial Rooftop PV Arrays, Designing for Fire Codes_Page_01

Designing Commercial Rooftop Solar Projects for Fire Code Compliance

Solar in the United States

We were updating the map on our webpage recently to show the locations where we are currently working and where we actively hold licenses in the the United States.  The result is this remarkable map below:

Solar Across the USA
Solar Across the US

The Past

It was only 5 years ago (in 2009) when we were actively licensed in about a dozen states and the lion’s share of our work was focused in California. Electricity is so expensive in California and our solar resource is so plentiful that solar energy makes great business sense when the effects of tax credits are entered into the equation. Plus, most of the solar projects we worked on in 2009 were rooftop 100kW to 500kW load-side connected projects.

The Present

As Jimmy Cliff once said, “The present is a gift, that’s why it’s called present.”

Today, we have found the 1MW to 20MW solar farm to be making up a large percentage of our engineering and design work. Solar carports have become increasingly common at schools and corporate campuses as rooftop space is often limited or obstructed. Because we’re paying well under $1.00/Watt for solar modules that will last thirty years or more, customers can afford the additional materials associated with fabricated steel carport structures and keep their investment returns under their hurdle rates.

It is also evident from the map that we have grown as an EPC (Engineering, Procurement and Construction) contractor to provide a single seamless point of responsibility for our customers.  The benefits to our customers has been the ability to deliver full multidisciplinary engineering and construction activities simultaneously. Thus, we are able to accelerate solar Commercial Operation Dates (CODs) and interconnection schedules far beyond our that of our competitors. Given the pace of this industry and the challenges from many directions, we needed to help our customers make projects happen at a pace greater than every thought possible. Since 2009, we have added more Project Engineers, plus Project Managers, Construction Managers, and we built a Civil Engineering team thanks to our dedicated staff. The rate of change in the renewable energy industry is nothing short of dizzying. But that’s what is required when you’re trying to move the needle from the solar energy industry’s current 0.57% makeup of nation’s energy production.

The Future

The US federal tax credit incentive is set to reduce from 30% to 10% for businesses on Jan 1, 2017. The reduction in this incentive is sure to change our map of active projects and licenses. However, we fully expect the solar energy market to continue growing and making up a larger dent in the global energy mix despite the many hurdles this industry faces. We need an energy mix, no doubt, but we also need a clear priority. That priority is clean energy.