Earth & Sky

The following individual was interviewed for today’s show. Our thanks to:

William W Nazaroff Professor of Environmental Engineering University of California, Berkeley Department of Civil and Environmental Engineering Our thanks to the following individuals and institutions who assisted in the preparation of this script: William Glaze Editor Environmental Science and Technology University of North Carolina Chapel Hill NC Professor Michael Overcash Department of Chemical Engineering North Carolina State University Raleigh, NC The following books, articles and web sites were used in preparing this script: Smart Communities Network (US Dept. of Energy) Consumer Energy Information (Office of Energy Efficiency and Renewable Energy – Department of Energy)

Ask an Energy Expert – specific questions about energy use (Office of Energy Efficiency and Renewable Energy – Department of Energy)

Author’s Notes: Bill Nazaroff [24.33]: I’m on the one hand, an academic, but I have a family and we make choices about what we do and we’re still trying to sort this out in terms of our own lives and we’re not kind-of at the position we need to be, if you projected how we live to the whole society to say “this would be good enough.” There’s still more that my family and I need to do. Dr. Nazaroff’s analysis of listener question: Which is more efficient and environmentally friendly, to run a quart of water down the drain waiting for it to get hot or simply microwave just a cup full? Comparing apples to apples, rather than to oranges It is not possible to directly compare two things measured differently and decide which is better. We must compare them on a consistent basis. In the way this question is phrased, we could consider several different bases: cost, time, energy use, water use, environmental impact, health to users. Hot Water from the Tap I’ll assume for this analysis that the hot water supplied at a residential tap is heated by means of natural gas water heater. Commonly, the water heater maintains hot water constantly in a 30- or 40- gallon tank. The water is delivered on demand through pipes. The response delay is caused by the need to flush cold water from pipes with hot water from the tank. The energy required to heat the water is not used with perfect efficiency. For a gas-fired heater, losses are associated with the release of warm combustion gases up the flue, with heat losses from the standing hot water in the tank, and with heat losses from the pipes. The actual efficiency for any specific task is difficult to assign because it depends on the pattern of use. I’ll assume here that the losses for heating and storing water represent 25% of the energy content of the natural gas. Microwave Oven A microwave oven takes electrical energy and converts it into electromagnetic radiation (microwaves) that is contained within the oven. Water in the oven is heated, because molecules absorb the microwaves. The efficiency of a microwave oven is estimated at 50% (http://www.howstuffworks.com). Performance Comparison The following table compares hot water from the tap with microwave heated water for several attributes that are readily quantified. The results indicate that the tap-delivered hot water offers better performance for cost and time, but that microwave heating uses less water and less energy. Table. Comparison of Properties of Delivering a Cup of Heated Water Attribute Tap Hot Water Microwave Heating Water use 5 cups (= 1.2 L) 1 cup (= 0.24 L) Primary energy use for heating 0.40 MJ a 0.34 MJ b Cost $0.0025 c $0.0037 d Time ~ 15 s ~ 2 min a Assumes 75% efficiency of the hot water heater and that all of the water used must be heated from 20 °C to 80 °C. b Assumes 50% efficiency of the microwave oven and that the water must be heated from 20 °C to 80 °C. Assumes that the electricity is generated at a natural-gas fired power plant with a conversion efficiency of 35%. c Based on a cost of $0.5 per therm and a water cost of $1.37 per 100 ft3 d Based on a cost of $0.12 per kWh and a water cost of $1.37 per 100 ft3 Additional Considerations: A comprehensive analysis would consider the life-cycle impacts of the entire system. So, for example, we would have to evaluate the energy use in treating and delivering drinking water. Also, we should consider the resource implications of manufacturing water heaters and microwave ovens. It is not a good idea to drink hot water from the tap. One reason is that any contamination in the pipes would tend to be released into the hot water much more readily than into cold water. One particular concern in old houses is that lead-based solders were used and the lead levels in hot water from the tap could be elevated. Don’t Sweat the Small Stuff Everything matters, but some matter much more than others. It doesn’t make sense to worry about negligibly small issues. Is this a small issue? I would argue that it is, on the following basis. Per capita use of drinking water in the United States is about 150 gal (550 L) per day. The savings of 1 L by choosing to microwave your water vs. letting the tap run represents a very small fraction of this total. Much larger savings would be realized by landscaping with native plants, and using low-flow showerheads and water-conserving toilets. Per capita energy use in the United States is roughly 1000 MJ per day. The energy used to heat a cup of water represents only 0.03% of this amount. If we are serious about the environment, then let’s focus on the big issues in energy conservation: taking fewer trips with private vehicles, driving more fuel-efficient cars, using energy-efficient lighting, and turning the thermostat down a few degrees in the winter and up a few degrees in the summer. How Do I Get My Hot Water for Drinking? I take cold water from the tap and heat it in a teakettle on an electric range. I never drink hot water from the tap.