To gain insights into the scope for “green” nanotechnologies, it is useful to first consider some broad energy-related issues. Energy technologies are commonly grouped into two main classes, both of which are covered extensively in this book:
- Technologies for production, supply, and storage of energy, and
- Technologies for reducing the demand for energy
Most people are more or less familiar fuel-efficient cars. But what about the importance of the types of windows we install, and the impact of novel paints and new thermal insulators, perhaps including phase-change materials? What can we accomplish with new passive cooling technologies using the sky as a heat sink, new structural materials and better manufacturing methods, superior energy storage techniques, and improved motors?
Supply and demand functions can be blurred or integrated in some emerging nanotechnologies. A simple example is a glass roof with integrated solar cells, which provides energy but also reduces the energy demand via the provision of daylight. A more sophisticated and futuristic example is multilayered “switchable” glazings in which some layers use the sun for providing power to optically switch other layers so that heat or glare in the building are kept down.
Energy supply for powering buildings, transport, and industry currently relies mainly on nonrenewable power sources including coal-fired plants and nuclear power stations coupled to electricity grids, natural gas via pipelines, and oil via refineries and tankers. Supply also includes a growing contribution from renewable sources including wind, water, bio-fuels and solar cells. Energy-demand technologies influence how much power is needed to run our homes, offices, schools, and shops; to operate factories, mines, waste treatment plants, and farms; and to transport people and goods, locally or across the world.
Solar radiation, wind, waves, and other sources of renewable energy are dispersed and intermittent regarding watts per square meter at any time. In contrast, the power density exiting the core of a nuclear reactor, or in a coal-Hred high-pressure steam vessel used to drive a steam turbine, is orders of magnitude larger and relatively steady in time. Wind, waves, and solar radiation arc thus not good for base-load power unless cost-effective energy storage is available or power networks cover very large distances, which is why these renewables are sometimes seen as top-ups with a cap on the percentage they can supply.
Alternative energy can be captured, converted, and stored in useful amounts. For this to work, large areas are needed for wind turbines, wave-powered generators, solar thermal collectors, or solar cells, and efficient storage is essential if these sources are going to be used for supplying base-load power. Storage efficiency is measured by weight or volume per unit of stored energy, and storage must enable us to deal with both the long-term (season) and short-term (hour or minute) dynamics of renewable energy sources. Fortunately, the areas and volumes of collectors that are needed are not impossibly large. For example, even with existing solar technology, two to three square meters are enough to supply a typical home with hot water, and its south-facing roof area (north-facing, if in the southern hemisphere) is usually more than sufficient to cover its electrical power needs, which generally scale with floor area and, hence, roof area.
In fact, it was the raised interest in solar energy during the oil crises of the 1970s that gave the first real boom in green nanotechnology and led to research on optical and thermal properties of nanoscilc metal embedded in insulating oxide layers and of nanostructured metal surfaces for use in flat plate solar collectors. And today (2010), again, concerns about the oil supplies and energy security are key drivers for new solar energy research. But this time we have, in addition, the even more significant threat posed by global warming. Fortunately, nanoscience has come a very long way since the 1970s, and has put a dazzling array of new experimental tools at our disposal, especially for imaging at tiny scales. There have also been many advances in physics and chemistry for understanding the growth of nanostructures and their properties. The emergence of nanoscience as a popular mainstream discipline during the late 1990s and 2000s is very fortunate, given the coincident realization that we must deal urgently with the energy challenge.
In addition to the well-known and intensely discussed threat from global warming, there is another less widely appreciated warming effect emerging from heat evolution in cities. This effect is of growing importance since not only is the world’s population increasing but the portion living in big cities is going up and already surpasses 50%. In particular, the growth of megacities leads to strong “urban heat islands” which contribute to the need for energy spent on cooling. For the specific case of Greater London, the urban cooling load was estimated to be 25% above that of surrounding rural areas and will be relatively higher still as the climate warming continues.
This book is devoted equally to the ways nanoscience can change the demand and supply sides of the energy equation. It is in on the demand side where most progress in carbon dioxide abatement is feasible in the short term. The potential and relative ease of demand-side improvement have been both badly underestimated and widely ignored due to entrenched practices. New materials and new science, if adopted, could impact energy savings and lifestyles to such an extent that, in 20 or 30 years, people will look back on current practices in bemusement. If we do not move quickly, however, the looks back may be looks of anger. Encouraging signs of changes have emerged recently, but education, more stringent regulation, and ready availability of new products are needed if these changes are going to influence how buildings are constructed and operated. Easy retrofits must also be available. Better choices of materials for constructing, insulating, and coating our build-ings and cars indeed can save vast quantities of energy. Coupled with environmentally aware building design and sensors and controls, nanomaterials may enable buildings that arc nicer to occupy, healthier and safer, and require only a fraction of their present power.
Some topics in this book have both supply and demand aspects — for example, lighting and cooling. The topic of lighting, involves the natural daylight resource plus lamps and light fittings. Daylight, in contrast to solar energy, is so abundant that one normally has to limit the amount that is let in. Unfortunately, practices went too far in its exclusion during the last 50 years due to an infatuation with new lamps. While lighting and cooling are energy-demand technologies and daylight is a natural resource, it is conventional to consider daylighting also as an energy-demand issue. This is so because it is used passively and enables a reduction in power for lighting while also improving the attractiveness of building spaces. It is important to first understand daylight as a natural resource. We shall see that it is so abundant and strong that in many climates its management is a challenge, but one which various nanostructures are well up to in windows and skylights. Savings on cooling might simply involve reducing the solar heat gain into a building by passive means to cut down on energy for air conditioning, or savings may entail active devices to tap into the environment and use radiative cooling toward the clear night sky to store up “coolness.” We will consider both lighting and cooling as demand-side technologies, even though sky cooling uses a largely untapped natural resource: the deep cold of outer space.
For convenience we will thus limit the term “supply” in the context of renewables to solar electrical power, solar heat, wind, waves, and renewable fuel sources, along with associated storage techniques. However, it is important to keep in mind that the environment also supplies abundant useful energy in other forms, especially natural light and the ability to pump a lot of heat away at night using little or no power. Tides, waves, and geothermal sources are also renewable resources, while local breezes for cooling at night are useful passive aids for saving energy in many populated locations, especially those near the seaside.
Studies on the various future contributions to reduction and stabilization ot CO2. emissions indicate that the largest single impact is expected to come from demand-side reductions, though overall there will be about equal contributions from all renewable energy sources combined and all energy savings technologies combined. The importance of energy efficiency is emphasized also in the most recent (2009) studies on enerev in the world. Efficient coal burning, gas, nuclear power, and carbon sequestration also will be needed to stabilize the CO2 content in the atmosphere.
Technological developments must take into account the costs and the benefits of investing in a product or system, but cost and energy savings are not the only factors that influence the decision to buy. The purchasers’ choice set—that is, the competition—is the starting point, and they will consider a variety of attributes of each product on offer. Novelty, durability, functionality, aesthetic appeal and design, impact on lifestyle, ease of installation, environmental impact, and government incentives are additional factors affecting the choice of energy technologies. The situation is changing somewhat, though, with the market for “green” products moving from niche to mainstream. The competition is thus shifting to the differences among “green” products themselves, and this leads to improved performance, lower cost, and elimination of products with poor quality. For example “green” buildings fulfilling certain standards in the United States command ~6% higher effective rents (adjusted for building occupancy) than comparable “nongreen~ buildings, and the selling prices of the green buildings are higher by ~16%.
If one cannot afford both solar power and better energy efficiency, then a choice has to be made. The energy savings per dollar invested is one starting point for comparison. tens by a factor between 20 and 40 to the benefit of the former option for many building types. Unfortunately, comparisons of this kind are rarely made. Devices such as solar cells are more “glamorous” and are seen by many scientists as a larger challenge, so they currently attract much more R6cD funding. This is a pity since nanomaterials for the demand-side effort also involves challenging new science, and, more importantly, offer vastly more energy savings per dollar invested in the near term. Both energy production and energy saving are needed, and it is the authors’ hope that one influence of this book will be to create a more balanced investment of scientific and technical efforts between these two components.
The primary attraction is not related to energy savings but to the impact daylight has on the functionality and appeal of interior spaces. The energy saving challenge is not to conserve on lamp use (though electricity indeed is saved) but to achieve better use of daylight without excessive heat gain or loss. A “multifunctional mindset” is clearly needed here, and we will see that nanotechnology is great for achieving multifunctionality. Examples of windows possessing four to five attributes that add appeal in addition to the benefits of daylight and a view will be discussed. Pointing these out will influence the customer’s choice and should be part of marketing efforts.