(last amended March 10, 2003)
Several times I have been asked, "if you had a fixed amount of money to spend on a pro-environmental initiative at Swarthmore College, what would be the best one?" This question is a difficult one to answer, because the College affects the environment in many ways. Among these are:
While difficult to trade off among these various different effects, I would recommend in principle that investments with the biggest bang for the buck are to be preferred. In particular, investments that reduce (or slow the growth of) energy use are the "low-hanging fruit" that we should pick first, since these translate directly into carbon not put into the atmosphere and into dollars not spent unnecessarily. Short payback times and large savings in life-cycle costs should attend energy-efficiency initiatives.
Comparing items 2 and 3 above, one can see that forgoing a MWh of electricity saves $81.21 while forgoing a MWh of hot water (or the heat equivalent of building heating) saves $27.30. Thus saving electricity is far preferable in terms of cost savings than is fuel oil. Environmentally speaking, since PECO is mostly supplying electrical energy from coal-oil-nuclear, the environmental damage resulting from a MWh of electricity is probably greater than that from obtaining a MWh of heat by burning No. 6 fuel oil (this is partly because the efficiency of turning fossil fuels into electricity is about 40%, while even including line losses and incomplete combustion, the efficiency of turning fuel oil into steam is over 60%).
Lighting is a good place to start, since it is more easily quantified. Consider the EXIT signs in most (older) buildings. They use two 20-watt incandescent light bulbs, which are always on (by law). In electricity use alone, therefore, each EXIT sign burns 40 watts continuously, or 350 kWh each year (for which we pay $28.42; see item 2 above). The bulbs last about one year, and cost $1.91 each to replace (not including labor, which is considerable but harder to put a dollar amount on), for a total of $32.24/year per EXIT sign. The newer, LED-based exit signs use only 4 watts continuously, with no bulb replacements necessary (all the ones in Kohlberg are examples of the newer kind). They cost $23.75 each, and so have a nine month payback time: after nine months, their cost is recovered in electricity saved relative to the incandescent variety, and everything after that is gravy. With a likely lifetime of 20 years, each LED sign would have saved $512 if it were not for the Present Value Modifier, which reflects the fact that savings now can be invested at the average discount rate (assume 8%; Mark Kuperberg, personal correspondence) and so the savings compound. The result is a savings of about $5027 per sign over that sign's lifetime. Swarthmore has over 400 EXIT signs on campus, for a total savings (over 20 years) of $2,010,800 (approximately $100,000 saved each year). Where else can you invest $9,500 and get over two million dollars twenty years later? (Note that this result assumes the cost of 1000 kWh to remain at $81.21 over 20 years. If the price of delivered electricity increases, as it is expected to, then the savings are even larger).
What about hot water? If we assume that the water begins at a cold temperature of 10 degrees Celsius (50°F) then the total energy to warm it to temperature T is J = X gallons/minute times Y minutes times (T-10) degrees Celsius times 15,824 joule-gallon/°C. Thus a hot shower (40°C) lasting 30 minutes at a showerhead flow rate of 2 gallons/minute uses up 28.5 million joules of energy (7.92 kWh). Assume for the moment that the water is heated using electricity. Each MWh of electricity costs the College $70, and is equivalent to 3.6 billion joules. Thus the hot shower costs $0.55 in electricity, and $0.42 in water (including sewer fees), for a total of 97 cents. Throw in the cost of pumping the water and you can consider that nice, long, hot shower to cost a buck. However, if 1400 students take such hot showers daily, the annual cost is a half-million dollars. Now, probably students do not take such long hot showers, at least daily, and certainly there are far fewer students on campus during the summer taking showers. So perhaps a quarter-million dollars a year is a better estimate of the cost of showers over a year. If the hot water is derived from burning No. 2 fuel oil, however, the costs are much less (and the savings correspondingly lower):.
How does this compare with EXIT signs? A 40-watt sign would have to burn for 198 hours (8.25 days) to use the same amount of energy as the long hot shower. To save the $16,000 each year that 400 LED signs would save, the average duration of each hot shower would have to be reduced 3 minutes, to 27 minutes on average. Alternatively, the flow rate of 2 gallons/minute could be lowered to 1.8 gallons/minute, or the temperature dropped from 40°C to 37°C (body temperature). However, these are quality-of-life issues for most people, and few students would want to take tepid 98.6°F showers of short duration just to save money (or energy). EXIT signs that meet building codes and are reliable and effective do not affect daily life very much. So would be any continuous use of electrical power, including fans, lights, and electric heaters that are always on. Like the dripping faucet, the total costs mount due to duration, even though the rate may be slow.
So EXIT signs are the "low-hanging fruit" of energy conservation for Swarthmore College, followed by all incandescent bulbs that are routinely left on. The payback time for occupancy sensors that would turn off unnecessary lights is relatively short (and easily verifiable), and that would be the next item on a theoretical agenda for cost/energy savings.