Modeling and Design of Net ZEBs as Integrated Energy Systems - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 94

CHP systems using biofuels can be combined with solar systems (such as
solar absorption cooling), thereby taking advantage of both options to reach
net-zero energy consumption while reducing peak demand for electricity
from the grid.
Thermal Efficiency
Heat engines are subject to the theoretical efficiency limits of the Carnot
cycle. When the fuel is natural gas/biogas, a gas turbine based on the
Rankine cycle is often used. Mechanical energy from the turbine drives an
electric generator. The low-grade (i.e., low-temperature) waste heat rejected
by the turbine can be used for space heating or cooling (using the heat to
drive an absorption chiller).
Total efficiency in a trigeneration system is given by
Typical trigeneration models have losses as in any system. The energy
distribution given next is represented as a percent of total input energy
Microcogeneration devices producing electrical outputs less than 15 kW
haveahighpotentialtodeliverenergyefficiencyandenvironmentalbenefits
in a Net ZEB. The concurrent production of electrical and thermal energy
from a single fuel source using fuel cells, Stirling engines, and
internal-combustion engines can reduce energy consumption and
associated greenhouse gas emissions, as described by the work of IEA/
ECBCS Annex 42 (Beausoleil-Morrison, 2008). Numerous manufacturers
are actively developing microcogeneration devices (Onovwiona and
Ugursal, 2006). The effective design of HVAC systems that can exploit and
effectively utilize the thermal output of microcogeneration devices is critical
if the benefits of microcogeneration are to be realized in a Net ZEB.
Furthermore, research is required to develop effective control strategies to
dispatch microcogeneration devices and associated HVAC components to
match their performance with the low thermal and electrical demands of a
Net ZEB. Small-scale CHP systems could possibly be combined with BIPV
systemsandbiofuelstoreachnet-zeroenergywhilereducingpeakelectricity
demand from the grid since CHP is dispatchable. Studies showed that a