16-0209 v2.X Net Positive Emissions Reduction Strategy

Question:

We are seeking confirmation that electrical energy from a photovoltaic system exported to the electrical grid to offset energy used from a hot water district energy system (DES) is an acceptable transition strategy. This approach results in a net-positive carbon impact benefitting our environment. See previous Dialog entry for project and DES details. We understand that LBC’s expectation is that the building send energy back to the DES equal to that consumed over the course of a year. For a heating water system, that would mean utilizing a solar thermal system to send hot water back to the DES. Although engineering such a system is possible, we have found that summer heating demand on the DES is often so insignificant as to make net-zero energy unachievable (as is the case for our building/campus). Even if net-zero energy were achievable, the resultant carbon emissions would be the same, net-zero emissions. Offsetting the thermal energy from the DES with electrical energy from a photovoltaic system will provide “net-positive” carbon emissions for the same amount of energy consumed. Our research has shown that the local electrical grid, MROW, has an emissions factor of 1433 lb CO2e/kWh (420 lb CO2e /MBtu), whereas wood biomass has an emissions factor or 43 lb CO2e /MBtu (1). So, production of 1MBtu of heat from the biomass DES would create 43 pounds of CO2e emissions; and production of 293 kWh (1MBtu) of electricity from a photovoltaic system would offset 420 pounds of CO2e emissions from the electrical power plant... a net-positive emissions reduction of 377 pound of CO2e emissions. Even with the high COPs associated with heat pumps (if they were to be used), emissions from biomass heating is still less than a third of that from a ground-source heat pump system. In summary, our proposed energy strategy is: A solar thermal system to provide a significant portion of the domestic hot water and space heating; A biomass-fueled DES to provide for the remaining (peak) domestic hot water and space heating needs; and A solar photovoltaic system to provide net-zero electrical energy as well as offset the thermal energy supplied by the DES. Emissions calculations attached. Source: (1) Emission Factors for Greenhouse Gas Inventories, U.S. Environmental Protection Agency, http://www.epa.gov/sites/production/files/2015-12/documents/emission-factors_nov_2015.pdf

Answer:

A biomass fueled hot water DES system is not acceptable as part of a Living Building energy strategy due to LBC dependence on combustion. In addition, carbon emissions calculations are not pertinent to I-06 Net Positive Energy v2.1 calculations. Projects can be tied to an electrical grid that includes combustion in its fuel mix if the project produces, on an annual basis, via onsite renewables, an equivalent amount of electricity to the amount it consumes. Tying into a combustion based system for hot water and providing PV as a calculated carbon emissions offset to a separate electrical system is not allowed.


Post ID 4275

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