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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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.

justshutit

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