Heliotactic Press

Interdisciplinary exploration of solar energy conversion, photovoltaics, and integrative design, and scientific philosophy.

Sustainability and Technology in the Habitable Environment 2010/01/02

Filed under: education,Environmental Technology,sustainability — nanomech @ 14:31
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In the USA, we  divide energy demand into four economic sectors: Industry, Transportation, Commercial Buildings, and Residential Buildings. By combining the Commercial and Residential sectors, we observe that 41% of our energy demand is derived from building environments (the habitable environment), and about 40% of our carbon emissions are derived from buildings. Additionally, Americans have been observed to spend 90% of their time indoors. This is a big chunk of power that we can work to make more sustainable! So what is sustainable energy technology, and why should we even use it?

Energy Flows (supply and demand) USA 2008

As seen in the figure, the majority of our energy supply has been from unsustainable sources (with the loose exception of hydropower). At some point I would like to provide a similarly enhanced figure that demonstrates the energy losses associated with supply and demand (for the curious: see here).

In terms of energy technology development and entrepreneurship, we may establish four generic divisions: energy supply, demand, storage/capacity, and usage. But energy technology development has a historical balance to be made with design, planning, policy and regulation. So why should we pursue sustainable energy solutions at all, or what is our sustainability ethic? Sustainability in planning and international policy can be addressed using the 1987 UN document Our Common Future (derived from the 1987 Brundtland Commission). It’s not the end-all document, but I would call it a good start for discussion and a must read for those learning about the history of modern sustainable energy solutions. The following list of general principals, rights and responsibiliteis comes directly from the Brundtland Commission report, Annexe 1:

Fundamental Human Right

1. All human beings have the fundamental right to an environment adequate for their health and well being.

Inter-Generational Equity

2. States shall conserve and use the environment and natural resources for the benefit of present and future generations.

Conservation and Sustainable Use

3. States shall maintain ecosystems and ecological processes essential for the functioning of the biosphere, shall preserve biological diversity, and shall observe the principle of optimum sustainable yield in the use of living natural resources and ecosystems.

Environmental Standards and Monitoring

4. States shall establish adequate environmental protection standards and monitor changes in and publish relevant data on environmental quality and resource use.

Prior Environmental Assessments

5. States shall make or require prior environmental assessments of proposed activities which may significantly affect the environment or use of a natural resource.

Prior Notification, Access, and Due Process

6. States shall inform in a timely manner all persons likely to be significantly affected by a planned activity and to grant them equal access and due process in administrative and judicial proceedings.

Sustainable Development and Assistance

7. States shall ensure that conservation is treated as an integral part of the planning and implementation of development activities and provide assistance to other States, especially to countries of the global South, in support of environmental protection and sustainable development.

General Obligation to Cooperate

8. States shall cooperate in good faith with other States in implementing the preceding rights and obligations.

Looking over this entire document, I find interesting underpinnings driving sustainable planning and design of energy flows. It is interesting that the argument here is much more than just a caution against climate change and loss of habitable space. Wow: inter-generational equity and ecosystem maintainence–heavy topics to consider. Additionally, the concept of Managing the Commons in Ch. 10 suggests an international valuation of common human resources that fall outside of national jurisdictions, but have critical importance due to ecological and economic interdependence.

And back to the technologies for the habitable environment:

For sustainable energy applications, an energy supply technology might take the form of algae-based biodisel production, wind power, or solar hot water (we typically call this division renewable energy). An energy demand technology addresses the management of the demand side of energy use (we typically term this division energy efficiency), specifically including efficiency in appliances and HVAC systems, weatherization of homes, and new lighting designs. Energy storage and capacity technologies are often lumped with other divisions, but they do include a significant share of tech: water tanks, phase change materials embedded in the walls, green roofs, batteries and ultracapacitors, and even the electrical power grid (to a certain extent). Finally, energy usage technologies are some of the more interesting developments of late, as they take advantage of modifying human behavior, just like the “Prius (or Honda Civic) effect”. Humans work well when given feedback (it’s like a game!), and much of our energy Demand (in the generic sense of the figure above) can be dramatically shifted toward reduction due to smart systems that inform you instantaneously as you consume energy. We used an amazing system for live monitoring by the company Noveda for the Natural Fusion home in the 2009 Solar Decathlon–very very cool technology.

We often hear the popular media discussion on buzz terms of “renewables” and “energy efficiency”. I have found that the innovation in the field of sustainable energy for the habitable environment is developing sufficiently to merit this new subdivision of scope. Perhaps this will even develop new discussions to refine the divisions for useful application in industry and transportation sectors.

 

Photovoltaics: Levels of Irradiance 2009/01/05

Filed under: Environmental Technology,PV Science,Uncategorized — nanomech @ 21:27
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Let’s talk about light interacting with a semiconductor to yield electricity. Today’s topic is to distinguish between low levels of irradiance and high levels of irradiance. Effectively, we are asking for an estimate of the concentration of photons being delivered from a high energy source to a low energy absorber/collector.

When we say low levels of irradiance, we are estimating a scale of light concentration that is typical of the diffuse and direct component of unconcentrated “global” or “total” solar radiation, or the light from a standard incandescent lamp or fluorescent lamp. This could be anywhere <1000 mW/cm2, or 10x the sun’s concentration (remember, this is just a crude scale, not a hard and fast rule–don’t take this back to your classes). The standard for testing solar cells inside the earth’s atmosphere is called Air Mass 1.5 Global (AM 1.5G), because the light from the sun passes through 1.5 lengths of a generic Earth’s atmosphere to generate a convenient irradiance of ~ 100 mW/cm2. Low levels of light such as this provide a sufficient number of photons (packets of light) to excite the electrons into an unoccupied level of energy (the conduction band). However, the population distribution of the majority carriers does not change significantly. That’s okay: the key player in a photovoltaic absorber is the minority carrier (n-type semiconductor: a hole; p-type semiconductor: an electron), and the population of minority carriers does change significantly with light absorption. Minority carrier transport gets the job done, in fact, because they are the limiting rate in the absorber reactor. You can find out more about charge carriers and carrier transport in the Photovoltaics CDROM from Honsberg and Bowden, Chapter 3 (although it doesn’t work completely for Macs, sadly)

What is high irradiance? You’ve heard the warnings about strong lasers pointing into others’ eyes? A laser is a coherent, collimated light source (the photons’ waves are in phase and heading the same direction), such that the photons can be very concentrated. If sufficient numbers of photons are absorbed by a semiconductor, the population of photoexcited charge carriers can be much greater than the majority carriers, and there a population inversion occurs, leading to stimulated emission (Light Amplification by Stimulated Emission of Radiation).

The photons from light bulbs and suns are neither coherent nor collimated, although they can be concentrated significantly to potentially cause a population inversion and stimulated emission (yes, there is the possibility for a solar laser). However, before that stage there are other phenomena that occur, making it a bit more complicated.

Concentrating cells allow an increased flux of photons to the smaller receiver/absorber using a larger aperture to collect the solar light. The geometric concentration ratio is the ratio of the area of an aperture to that of the absorber (C=Aapt/Aabs).1,2 For a perfect concentrator (as a point on the surface of Earth), the radiation from the Sun on the aperture-receiver assembly is only a fraction of the total radiation emitted by the Sun, given a half-angle subtended by the Sun of 0.27°. Assuming a blackbody, the absorber would have a maximum theoretical concentration ratio of 45,000 (for a circular concentrator) or 212 (for a linear,trough concentrator).1 The higher the concentration,the higher the photon flux (including increasing temperature),and the more precise the optics of the collector must be to deliver. This is an extreme energy flux for any semiconductor. Under high illumination levels, one will observe a decrease in minority carrier lifetimes and related diffusion path lengths. However, 45.6% of the suns power is contained in the infrared band (the part that makes things “hot”). Thermally, an imaging concentrator (C>> 10; analogous to camera lenses) can produce temperatures from 500 to 1500 °C at the absorber.2 This increased temperature can be used to drive thermal work (steam generation) or thermophotoelectrochemical reactions for concentrating solar power (CSP, not to be confused with CPV), but is not necessarily good for photovoltaic performance. High temperatures tend to decrease the efficiency of a photovoltaic device. In particular, this is why members of the microelectronics industry are getting into the concentrating photovoltaics field (CPV)–they know how to cool superhot microelectronics, and will do the same with CPV devices.

It is so interesting to see how this is all a great spread of possibilities that one can derive from our nearest fusion reactor!

Text sources:
1. Rabl, A. Active Solar Collectors and Their Applications. 1985 Oxford University Press, New York

2. Duffie, J. A.; Beckman, W. A. Solar Engineering of Thermal Processes. (3rd Ed.) 2006 John Wiley & Sons Inc, Hoboken, NJ, USA.

3. Andreev, V. M.; Grilikhes, V. A.; Rumyantsev, V. D. Photovoltaic Conversion of Concentrated Sunlight. 1997, John Wiley & Sons Ltd, Chichester, England.

 

 
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