Embodied Energy and Solar Cells

Embodied Energy and Solar Cells

By Devon Holst, Member-at-Large for the GCI

Embodied energy is the sum of all energy consumed in the production of goods and services. Knowing the amount of energy something ‘embodies’ is useful when assessing the environmental impact of comparable goods and services as well as assessing the utility of technologies that produce or save energy. If a device intended to save energy embodies more energy than it will save over the entirety of its use, the product is considered to be unfavourable. A net energy loss would be the result of its application.

It is important to consider the embodied energy of renewable energy technologies to ensure there is a net energy gain. I am going to follow the production process of silicon solar cells as an example of how energy can be embodied into a product. To be effective, the embodied energy of a solar cell must be less than the total energy it produces. There are many processing steps needed to assemble a solar cell where the embodied energy should be kept to a minimum. Some of the largest sources of embodied energy in silicon solar cells are described below.

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Silicon Processing (Additional embodied energy: 460 kWh/kg)

Carbothermic reduction of quartz sand (silicon dioxide) is used to produce metallurgical grade silicon. This process consumes 20 kWh/kg of metallurgical grade silicon produced. Metallurgical grade silicon must then be further refined to electronic grade silicon through a reaction with hydrochloric acid at 300 oC followed by treatment with hydrogen gas at 1100 oC. This process consumes 100 kWh/kg of electronic grade silicon. This silicon is then melted at 1400 oC and crystallized, consuming 290 kWh/kg of silicon single crystal. This form of silicon is suitable for use in a solar cell. After accounting for losses of material during each step, these processes embody 460 kWh of energy into each kg of silicon single crystal.1

Solar Cell Production (Additional embodied energy: 120 kWh/m2)

The single crystal of silicon is sliced into wafers with a multiwire saw resulting in a 40% to 50% loss as dust. Following this, a sequence of high temperature diffusion, oxidation, deposition, and annealing steps are performed. This adds 120 kWh/m2 ­­­of embodied energy to the solar cell.1

Module Assembly (Additional embodied energy: 190 kWh/m2)

A module consisting of a glass front panel, an encapsulant, the solar cell, copper ribbon, a foil back cover, and an aluminum channel is then assembled. 190 kWh/m2 of embodied energy is added during assembly.1

Support Structure (Additional embodied energy: 200 – 500 kWh/m2)

The module is then typically installed in a field or on a rooftop. In a field, the module needs to be supported by concrete, cement, and steel. Construction and materials add 500 kWh/m2 of embodied energy. Rooftops have an existing support structure reducing the embodied energy of this aspect to 200 kWh/m2.1

Miscellaneous Components

Beyond the former sources of embodied energy there are many other components in an operational solar cell. An inverter, wiring, and a battery are a few examples of these components. Depending on the components needed, this will add a variable amount of embodied energy.1

Devon_blog2Emerging technologies such as perovskites and organic solar cells often have much lower embodied energies than their silicon counterparts. Material processing methods and the amount of material necessary to produce a solar cell are a couple of the major factors that account for the difference in embodied energy of these technologies.1,2 There are, however, many other factors that make a solar cell viable for large scale energy production which when considered in aggregate currently favour silicon solar cells. It is likely that multiple solar energy technologies will thrive in the future as each has unique characteristics making one more applicable to a given situation than another.1,3

The energy payback time of a given solar cell is calculated by dividing embodied energy by energy output per unit time. This is the amount of time a solar cell must operate before it generates the same amount of energy as its embodied energy. Silicon solar cells have a 1.65 to 4.12 year energy payback time, while some organic solar cells and perovskites have energy payback times of less than half a year.4,5

Embodied energy is part of an even broader picture. A picture that captures the energy used to recycle or dispose of something and the energy associated with environmental impacts incurred through goods and services in any way. The picture is complex, but a deep understanding of it is necessary in order to make decisions that are conscious of the future.

I wonder how much energy I embody…

References:

1) Nawaz, I.; Tiwari, G. N., Embodied energy analysis of photovoltaic (PV) system based on macro- and micro-level. Energy Policy 2006, 34 (17), 3144-3152.

2) Anctil, A.; Babbitt, C. W.; Raffaelle, R. P.; Landi, B. J., Cumulative energy demand for small molecule and polymer photovoltaics. Progress in Photovoltaics: Research and Applications 2013, 21 (7), 1541-1554.

3) Snaith, H. J., Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. The Journal of Physical Chemistry Letters 2013, 4 (21), 3623-3630.

4) Espinosa, N.; Hosel, M.; Angmo, D.; Krebs, F. C., Solar cells with one-day energy payback for the factories of the future. Energy & Environmental Science 2012, 5 (1), 5117-5132.

5) Gong, J.; Darling, S. B.; You, F., Perovskite photovoltaics: life-cycle assessment of energy and environmental impacts. Energy & Environmental Science 2015, 8 (7), 1953-1968.

Image Sources:

  1. Solar panels (https://commons.wikimedia.org/wiki/File:SolarparkTh%C3%BCngen-020.jpg)
  2. Embodied energy (http://www.paveshare.org/library/embodied-energy)
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Just Shut It!

By James LaFortune and Shawn Postle, Members-at-large for the GCI

The Green Chemistry Initiative (GCI) and the University of Toronto constantly strive to reduce the environmental impact inherent to research in the Chemistry Department.  A major contributor to the department’s carbon footprint is its fume hoods, which are required to safely carry out experimentation with volatile solvents or reagents.  They provide a partially enclosed working space that draws air from the laboratory into the hood, thereby reducing the researcher’s exposure to chemicals. However, this requires replacing the removed air with acclimatized outside air, a process which consumes large amounts of energy in heating or cooling the air.

Fume hoods are designed with a height-adjustable glass pane (sash) that allows the researcher to work while keeping the space as enclosed as possible. There are two main types of fume hoods used in chemistry labs: constant air volume (CAV) and variable air volume (VAV).  With CAV, the air flow remains constant regardless of sash height, while VAV systems are designed to moderate air flow based on the sash height.  Each fume hood requires roughly as much energy as three American households when in use.  However, roughly 50% of this energy can be conserved in VAV systems if the sash is kept shut.  Given that the Chemistry Department has 123 VAV fume hoods, ensuring that VAV sash heights are minimized during idle periods is an effective strategy to reduce unnecessary energy consumption.1

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The GCI’s Just Shut It! Campaign, renewed this past summer for its third season since 2008, encourages graduate students to close VAV fume hood sashes during idle times and to minimize sash height when working in the hood.  Fume hoods are checked weekly by GCI volunteers to ensure compliance and users of fume hoods found to be in compliance are entered into monthly prize draws.  The past two Just Shut It! Campaigns (read more about the original campaign) were very successful, where compliance rates increased from 3% to 61% during the campaign. Since its reinvigoration over the summer, we have recorded similar compliance levels as previous campaigns.

What’s more, the Chemistry Department currently only uses VAV fume hoods in its Davenport wing.  However, it is exploring air systems renovations, including replacing CAV with VAV fume hoods in much of the Lash Miller wing.  These additional VAV fume hoods will further decrease our environmental impact.

This time around, we are looking to keep the Just Shut It! Campaign going permanently.  Gracious thanks to the Department of Chemistry for funding this campaign, especially Chief Administrative Officer Mike Dymarski.

  1. E. Feder, J. Robinson and S. Wakefield, International Journal of Sustainability in Higher Education, 2012, 13, 338-353.

Green Chemistry Principle #6: Design for Energy Efficiency

By Trevor Janes, Member-at-Large for the GCI

6. Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.

In chemistry (and in life) we need energy to do work. Every task we do in the lab requires energy: whether we’re using a Bunsen burner or weighing out a reagent or dissolving our favourite compound, in all cases we’re using energy in some form.

In the lab, we often need to change the pressure and temperature of experiments, and this uses a large amount of energy. Ideally, we would perform all reactions at ‘ambient’ conditions – room temperature and atmospheric pressure – in order to minimize energy usage.

In Video #6, Julia and David use an energy monitor to see help us see just how much energy is used by everyday lab equipment. They measure a vacuum pump, which is used to reduce pressure, and a hot plate, used to raise the temperature of a reaction.

Julia and David measure the power used by each instrument and calculate the monthly energy bill, comparing the cost and amount of energy to a regular household item like a TV.[1] By doing this they determine the financial impact of the energy requirements of lab equipment. A hot plate uses roughly as much energy as a TV, and a vacuum pump uses more energy than 3 TVs! Just like at home, minimizing the use of equipment in a lab, and turning off equipment when it’s not in use, will conserve energy and save money.

In an academic lab, the amount of energy and its associated cost is modest and may seem insignificant. But on the much larger industrial scale, energy/money savings are multiplied and energy efficiency becomes even more important.

We know that heating a reaction requires energy, but another energy-intensive aspect of lab work that occurs after completion of the reaction is the work-up. “Working up” the reaction means separating our desired product from the other components in the reaction mixture such as solvent and byproducts. We talked about this before in our post for Principle #5.

To remove solvent conveniently we use a rotary evaporator, commonly referred to as a “rotovap,” which involves the combined use of a heat source, vacuum pump, rotating motor, and chiller. The heat, vacuum, and rotation vaporize the solvent and the chiller condenses the solvent vapors into a flask for removal. If you’re curious, we also measured the energy used by the chiller component of the rotovap assembly (see calculations below). If left on all the time, the monthly energy bill for the chiller alone would be $15.60 – the same as 2 TVs – and that’s not including the other rotovap components. If we can develop chemical reactions that avoid solvent removal and/or simplify work-up, we can save energy and money.

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Our “Shut It” campaign encourages fume hood sashes to stay closed.

Later in the video, we were delighted to host special guest Allison Paradise, Executive Director of My Green Lab who joined us to highlight the importance of minimizing the energy used by chemical fume hoods. As the My Green Lab website explains, there are Constant Air Volume (CAV) and Variable Air Volume (VAV) ventilation systems.[2] In VAV systems, closing the fume hood sash allows the exhaust fan to run more slowly while maintaining a safe flow rate. By closing our sashes in VAV systems we can reduce energy use by 40% or more!

Turning off your TV after you’re finished watching it illustrates the idea behind Principle #6. Just like you care for the environment and save money by being energy efficient at home, we want to minimize the environmental and economic impacts of the chemical processes we do in the lab.

Energy Calculations:

Julia and David measured the vacuum pump to draw 360 W. If we kept it on for one month, this would be 259 kWh. In Toronto, the consumption-based cost of electricity is $0.108/kWh,[1] which makes the cost for one month of vacuum pump use $28.

360 W x (1 kW/1000 W) x (720 h/1 month) = 259 kWh/month

259 kWh x $0.108/kWh = $28

The hot plate heating an oil bath to 110 °C uses 100 W, which amounts to 72 kWh in one month. Using the electricity cost of $0.108/kWh again, the monthly bill for keeping the hot plate on at all times would be $7.80.

100 W x (1 kW/1000W) x (720 h/1 month) = 72 kWh/month

72 kWh x $0.108/kWh = $7.80

Not included in the video is the measurement of a rotovap chiller. This chiller circulates coolant that it maintains at -5 °C, which requires 200 W. This is double the power drawn by the hot plate and represents a monthly energy bill of $15.60.

References:

[1] Cost of electricity and household appliance energy usage, Toronto Hydro: http://www.torontohydro.com/sites/electricsystem/residential/yourbilloverview/Pages/ApplianceChart.aspx

[2] My Green Lab’s explanation of fume hood types and their energy consumption: http://www.mygreenlab.org/be-good-in-the-hood.html

Shut It Campaign

By Nadine Borduas, Member-at-Large for the GCI

If you happen to be visiting the Davenport Wing in the Department of Chemistry at U of T, you will notice along the side of each fumehood a ruler with hues of orange and grey with thumbs pointing up and down. Why? Well, because the fumehoods in Davenport are variable-flow fumehoods! In other words, the flow through the fumehood is dictated by the sash height. The higher the sash → the higher the flow → the higher the energy consumption → the lower the thumb. It also implies that if everyone had their fumehoods wide open, the energy consumption would actually exceed the energy of the constant-flow fumehoods in Lash Miller and invalidate the variable-flow technology as a green alternative. So, how did the GCI intervene to help ensure that variable-flow fumehoods were being kept at optimal heights?

We launched an ongoing "Shut It" campaign to encourage optimal fumehood sash heights!

We launched an ongoing “Shut It” campaign to encourage optimal fumehood sash heights!

Back in 2008, the Sustainability Office (SO) at U of T had run a campaign to “Just Shut It”. The campaign was launched to minimize the energy consumption of the variable-flow fumehoods in the Davenport Wing. Over the course of approximately a year and a half, SO ambassadors inspected the fumehoods and rewarded complying students.

There are two ways to comply: 1) if you’re not at your fumehood, it should be shut below 3 inches, and 2) if you are working at your fumehood, its height shouldn’t exceed 14 inches. The SO team analyzed the data and published the result of the campaign in the International Journal of Sustainability in Higher Education. Pretty impressive research!

What is most notable in this article, in my opinion, is a graph showing the compliance percentage over time. The sash height compliance during the campaign was about 80%, but eight months later, termed post-campaign, it was back down to 20%! The lack of compliance after the campaign may be due to the lack of incentives and to the quick turnaround of students in the department. So the GCI, a sustainable group of students that will hopefully be around for years to come, decided to launch a campaign, where you would have no “post-campaign” syndrome and consequently maintain high levels of energy savings.

With the help of GCI inspectors (Peter, Ian, Christine, Ran, and myself), we toured the Davenport Wing from July to September, stamping smiley faces on complying fumehoods. Kai Wan form the Morris Group was awarded a Tim Horton’s gift card for having perfect compliance over the fifteen inspections, and the Zamble Group won a free pizza lunch for having the best compliance on a group basis.

We hope the organic, bio-organic, and inorganic students of the Davenport Wing enjoyed the challenge and will continue to “just shut it”. Stay tuned for the continuation of this campaign in 2015 as well!