Figure 5.4 shows the most and least sustainable energy options for electricity and heat systems based on the industry’s weighting scheme. The estimated environmental impacts and costs among alternatives for all U.S. states can be found in Tables D.2-3 in Appendix D.
(a) (b)
(c) (d)
Figure 5.4 The most and least sustainable energy options for (a and b) electricity generation and (c and d) heat generation from industry’s perspective (PV-T: solar photovoltaics with thin-film panels, NG: natural gas generator, PG: propane generator, DG: diesel generator, NB: natural gas boiler, SHW: solar hot water, DB: diesel boiler, and AD: anaerobic digestion)
For electricity systems, the use of natural gas generators showed the lowest environmental
impacts across the U.S., due to low environmental impacts related to the production and use of natural gas. On the other hand, the economic benefits of alternative energy systems varied depending on incentive availability and local energy prices. As a result, the sustainability index for different energy systems was different across the states. For instance, compared to a solar PV system in Virginia, the environmental
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impact single score of a natural gas generator was lower (Figure D.1 in Appendix D). However, economic benefits with solar PV systems were higher than the use of natural gas generators (Figure D.2 in
Appendix D), resulting in higher SIs with solar PV systems in Virginia. Among the solar PV systems, the use of solar PV-T systems was the most economical option in all states, due to its relatively low initial costs (1.26 $/Wdc for solar PV-T, 2.80 $/Wdc for solar PV-M, and 1.68 $/Wdc for solar PV-P). Thus, the use of solar PV-T systems showed the highest SIs in most states (Figure 5.4-(a)), except for KS and WV. On the other hand, the use of diesel generators was the least sustainable options in most states, due to high environmental impacts and low economic benefits.
Tables D.4-5 in Appendix D show the rankings of sustainability for electricity generation options and heating options, respectively. The solar PV systems were the most sustainable electricity options in most U.S. states. However, in some states (14 out of 39) where there are no incentives available for solar PV systems and/or low relative local natural gas prices, the use of natural gas system showed a higher SI than a solar PV system. In CO, MD, OR, SC, TN, ME, MI, NY, and UT, the use of natural gas generators had higher SIs than a solar PV-M system, while the natural gas generators showed higher SIs than both solar PV-P and PV-M systems in IN, KS, WV, OH, and NE. Especially, the natural gas generators showed higher SIs than all types of solar PV systems in KS and WV due to low local relative natural gas prices (0.49 $natural gas/$electricity in KS and 0.24 $natural gas/$electricity in WV) and the lowest environmental impacts of natural gas among alternatives. Additionally, no incentives were available for solar PV systems in these regions (Figure 5.4-(b)).
For heating systems, the use of natural gas boilers generally showed the lowest single scores (environmental impacts) and the highest economic benefits among alternatives (Table D.5 and Figures D.3-4 in Appendix D). Tsilingiridis et al. 2004 also reported the use of a natural gas boiler as a substitute system of an electric heater showed less environmental impacts than the use of a SHW system. Thus, the natural gas boilers were the most sustainable options in most states, as shown in Figure 5.4-(c). In FL and HI, however, the use of SHW systems showed the highest SIs due to their low environmental impacts and high economic benefits from incentives available in these two states. Contrary to FL and HI, the use of
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SHW systems in AK resulted in the lowest SI among alternatives due to unfavorable geographical conditions (e.g., low annual solar insolation and cold temperature) (Figure 5.4-(d)). Among the alternatives, the use of diesel boilers was the least sustainable options in most states due to the high environmental impacts and low economic benefits. For the rankings of sustainability among heating options, the use of natural gas boilers had a higher SI than a SHW system in AL, AR, CO, GA, ID, IL, KY, LA, ME, MO, MS, NC, NJ, NY, OK, OR, PA, SC, UT, TX, VA, WA, and WV, while both natural gas boilers and propane boilers showed a higher SI than a SHW system in CA, CT, IN, KS, MA, MD, MI, MN, NE, OH, RI, TN, and WI (Table D.5 in Appendix D). All three conventional heating sources (i.e., natural gas, propane, and diesel) had higher SI values than a SHW system in AK.
The integration of anaerobic digestion as a backup with other heating systems resulted in a lower SI than that of a heating system without anaerobic digestion (Table D.5 in Appendix D). However, in some states, the use of diesel boilers with anaerobic digestion showed a higher SI than the use of a diesel boiler as a single unit. These states include AL, AR, CA, CT, FL, GA, HI, ID, LA, MA, ME, MO, MS, NC, NJ, OR, PA, SC, TX, VA and WA, which are classified as medium or large scale fish farms (> 5,000 metric tons/yr/farm) in Figure 5.1. In MA, the use of natural gas boiler with anaerobic digestion showed the highest SI value among alternatives (Table D.5 in Appendix D). This may be due to higher economic benefits with a large volume of biogas generation with available fish wastes, considering the trade-off between the initial costs of the anaerobic digestion systems and the amounts of economic benefits from energy savings through biogas generation. One exceptional case was found in RI, which has small scale fish farms (< 400 metric tons/yr/farm). In RI, the use of natural gas boiler with anaerobic digestion showed the higher SI than a single natural gas boiler as well as other alternatives. This is because the incentive for anaerobic digestion in RI (PBI: $0.2/kWh) helped increase the economic benefits of the hybrid system (i.e., a combination of a natural gas boiler and an anaerobic digestion system). However, the current incentives for anaerobic digestion in MN (PBI: $0.02/kWh) and NJ (Rebate: a 30% of initial cost) were found not sufficient to improve the economic viability of anaerobic digestion.
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