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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Recycling Perovskite Solar Cells

Recycling Perovskite Solar Cells

By Judy Tsao, Member-at-Large for the GCI

Solar energy is arguably the most abundant and environmentally friendly source of energy that we have access to. In fact, crystalline silicon solar cells have been employed in parts of the world at a comparable cost to the price of electricity derived from fossil fuels.1 The large-scale employment of solar cells, however, remains challenging as the efficiency of existing solar cells still needs to be improved significantly.

An important recent breakthrough the field of solar cells is the use of perovskite solar cells (PSC), which includes a perovskite-structured compound as the light-harvesting layer in the device (Figure 1). Perovskite is a name given to describe the specific 3-D arrangement of atoms in such materials. Even though the first PSC was reported only in 2009, its power conversion efficiency (PCE) has already been reported to exceed 20%, a milestone in the development of any new solar cells which typically takes decades of optimization to achieve.2

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Figure 1. Thin-film perovskite solar cell manufactured by vapour deposition (photo credit: Boshu Zhang, Wong Choon, Lim Glenn & Mingzhen Liu)

PSC has several advantages compared with traditional solar cells, including low weight, flexibility, and low cost.3 There are, however, several challenges that must be overcome before PSC can be brought to the market. The most common PSC to date includes CH3NH3PbI3 and related materials, which contain soluble lead (II) salts that are toxic and strictly regulated.

Interestingly, there has been a consensus in the literature that the lead content in the perovskite layer is not actually the main issue in the environmental impact of PSC production.4 Part of the reason for this conclusion is simply that the thickness of perovskite layer required would amount to less than 1000 mg of lead in one square meter of material. This value is only modest compared to lead pollution from other human sources such as lead paints or lead batteries.5

The main environmental concerns regarding PSCs appear to lie in the use of gold and high temperature processes during the manufacturing of the devices.6 It has thus been suggested that, in order to reduce the environmental impact of PSCs, recycling of raw materials is very important. In a recent study by Kadro et al., 7 a facile protocol for the recycling of perovskite solar cell was developed. The entire procedure takes place at room temperature and takes less than 10 minutes (Figure 2).

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Figure 2. Schematic process for recycling PSC components [7].

As it turns out, components of a fully assembled PSC can be extracted by sequentially placing the device in different solvents. Step 1 of the procedure uses chlorobenzene to remove the gold layer, while step two uses ethanol to dissolve CH3NH3I. This then leaves PbI2 to be the only component remaining on the device, which can be removed by just a few drops of N,N-dimethylformamide. It is also worth noting that the recycled materials can be fabricated into a complete PSC again without significant drop in performance.

Even though the discovery of PSC has only been made less than a decade ago, its potential in applications in photovoltaics has been underlined by numerous studies. It is especially gratifying to see that the environmental impacts of such devices are already under active research before PSCs are introduced to the market. While these studies have demonstrated that PSCs have low environmental impacts when properly recycled, there are other challenges still facing researchers in this field. In particular, the short lifetime of such devices needs to be improved to match that of traditional silicon-based solar cells. Nevertheless, the facile method of recycling PSCs without compromising the performance will certainly make them even more competitive than traditional solar cells.

References:

  1. Branker, K. et al. Renewable Sustainable Energy Rev. 2011, 15, 4470.
  2. Yang, W. S. et al. Science, 2015, 348, 1234.
  3. Snaith, H. J. Phys. Chem. Lett. 2013, 4, 3623.
  4. Serrano-Lujan, L. et al. Energy Mater. 2015, 5, 150119.
  5. Dabini, D. Phys., 2015, 6, 3546.
  6. Espinosa, N. et al. Adv. Energy Mater. 2015, 5, 1.
  7. Kadro, J. M. et al. Energy, Environ. Sci. 2016, 9, 3172.