We have seen in the foregoing lectures that fossil fuels are a non-renewable resource, we will run out of oil in ~50 years and coal (the only real plentiful fossil fuel) is dirty (S, metals) and Carbon-rich. So what to do to feed our energy hunger?
Look at renewable resources and curb that hunger through Conservation!
The following chart gives an overview of the total alternative energy
available and the potential extractable amount. Remember, in 2001 the world
energy demand is about 500 quad/year
| RESOURCE | TOTAL ENERGY (WATTS) | POTENTIAL EXTR. (QUADS/YR) |
| SOLAR (HEAT + LIGHT) | 1.7 1017 | 100,000 ? |
| WIND | 1.3 1015 | 3900 |
| HYDRO | 9 1012 | 85 |
| WAVES | 7 1013 | 80 |
| OTEC | 5 1013 | 60 |
| GEOTHERMAL | 3 1013 | 4 |
| TIDAL | 3 1012 | 2 |
One of the most promising renewable energy sources is solar energy and we distinguish
Direct Use
Passive buildings constructed to optimize solar heating, storage of heat in
Average heat received from the sun worldwide over 24 hours is about 240 W/m2, with much higher values in the tropics and much lower values on the poles. In Connecticut this value is about 100 W/m2. Imagine that average daily electricity demand in a CT household is about 20 kW-hour/day, which equals roughly 1000 Watt, you will need 10 m2 surface to capture that heat and probably 100 m2 to convert that solar energy into usable power. To satisfy US per capita total energy demand (about 10kW), we would need about 1000 m2 capture surface per capita, and with 250 106 people in the USA, that would require 250 109 m2. The USA has ~9000 109 m2 surface area, so about 3-5 % surface area covered by collectors would do the job. In cities, this would be a problem but in more rural and suburban areas, it may be possible without major disruption of lifestyles.
Wind - the total amount of wind power available is enormous and in Europe this is being harnessed at large scale. Some wind energy farms exist in Oregon and California, but it is small compared to Europe. The region off the NE USA coast has on average 700W/m2 potential windpower and a floating energy wind farm on George's Bank could potentially provide electricity for the major east coast urban centers.
Hydropower - About 25% of electric power worldwide
is generated by hydropower. Environmental problems are large, from displacement
to people to wiping out of ecosystems to weight induced earthquakes and
the dangers of natural earthquakes. In addition , some of the larger dams
in the world suffered from poor design. The 1964 Assuan dam especially
stands out with errors in the estimate of silting up, leakage and evaporation.
The farmers downstream also lack the annual natural fertilization cycle
from the clay and silt that the Nile used to bring and even the ecosystem
in the Mediterranean Sea is impacted by the lack of nutrients that the
Nile used to bring in.
Waves and Tides- little use has been made of these large resources, mainly because of technical limitations. Tidal energy generation stations occur in France and the UK, with dams that close of a tidal bay. The incoming and outgoing tide drives simple turbines and generates electricity.
OTEC- Surface waters in the tropics are about 20-25 oC whereas water at 1 km depth can be close to 5 oC. Any temperature difference will run a thermal to mechanical energy conversion contraption, but the thermodynamic efficiency will be very low. The Ocean Thermal Energy Conversion plants have low yield and must be build at a massive scale. It is envisioned that NH3 (ammonia) will be the working fluid, which boils at a low temperature. Large amounts of water will be pumped over heat exchangers with liquid ammonia, which will bil, drive a turbine and is then condensed again by the colder deep water that is pumped up. About 70% of the generated energy will be spent on the pumping of water that is needed. A mini OTEC ran off the coast for Hawaii for awhile, and no plans are in the works yet to design the massive plants needed to OTEC a viable resource.
CONSERVATION
In house conservation consists largely of using energy efficient household and industrial appliances, insulation both in hot or cold climates and new technologies in transportation. With the USA urban landscape largely taken up by suburbs, the idea of mass transit is still far away and we will have to look at more efficient cars. We discussed the hybrid technology (e.g., Toyota Prius), diesel engines and regenerative breaking. In general, energy storage can occur through electric storage (batteries), mechanical storage (flywheels) or chemical storage (H2 gas). Insulation involves the application of Fouriers Law
Q = k dT/dz
where Q is the conductive heatflux (W/m2), k = thermal conductivity (a material property) and dT/dz is the temperature gradient (or the temperature difference between two spots separated by a distance z). To reduce heat loss from houses in cold climates, reduce of course convective heat loss (seal everything up) and to reduce conductive heat losses we can either reduce dT/dz or k to make Q smaller. In the old days, we made dT/dz smaller by making very thick walls (z larger). Today we mainly make k smaller by using insulation. The R values on insulation are 1/k, and the higher the R values the better insulator we have. Usually, we apply glasswool where air is trapped between fibers. Air is a much poorer heat conductor than solids, so the net insulation value of the trapped air in the glass fibres is high. Double paned windows are another common energy saving mesure, where the double panes slightly increase z, but the biggest gain comes from the injection of a poor heat conducting gas between the two panes (usually Ar). So both dT/dz and k are adapted to reduce Q. The problem with all the insulation and draft removal and sealing up is the occurrence of more severe indoor air pollution (e.g. the Radon problem).