ASHRAE Journal - December 2019 - 42

SOLVING PROBLEMS

In this configuration, PV installations reduce peak
electricity load. This generally does not affect CHP sizing
because well-designed and efficient CHP systems are
currently designed for high load factors-where 85% or
more of the operating hours are cogenerating electricity and useful thermal energy to serve the site base load,
Sweetser said.
He said adding batteries to solar/CHP hybrid systems is
gaining traction. When designing CHP with PV systems,
Sweetser said it is critical to understand the concept of
transients caused by clouds.
"When a cloud passes over a solar field, electricity
production precipitously drops, which destabilizes an
islanded site, which can cause the CHP plant to drop
offline," he said. "Batteries are typically employed to
mitigate this impact."
"If you have PV operating with CHP in island mode,
you have to put in batteries. But again, you're not going
to put in batteries for a week's worth of photovoltaic
capacity. It's way too expensive. You put in batteries for
minutes or at maybe an hour at most, that type of thing,"
he said.
Foley said integrating intermittent renewable energy,
especially solar PV and CHP generators, has caused
problems when maintaining systems on-line during
fluctuating renewable energy output. He said the industry is dealing with this challenge, and batteries will help
resolve the issue.
Sweetser said in New York City, some CHP systems are
coupled with batteries only because tall buildings are
not the best place to economically locate PV systems.
Right-sizing the batteries and charging them at night
results in improved economic performance out of the
hybrid system consisting of a smaller-sized CHP plant
combined with batteries.

Wind Energy

Wind generally has a slower rate of ramp up/ramp
down rate, which is less of an issue when integrating
wind with CHP than dealing with the fast ramp speeds
required in matching PV systems with CHP, said Foley.
Wind has traditionally sought to partner with CHP,
which can help efficiently fill in the output valleys during daytime when loads are high and wind production is
low, Foley added.
"The interesting thing is wind power creates a lot more
energy at night than it does during the day because of

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D E C E M B E R 2 0 19

the various wind currents and temperature differences,"
said Sweetser. "And that's exactly when we don't need
the extra electricity because most people are asleep."
He said batteries are one way to store wind energy.
Another way is to create hydrogen from the wind energy
that is storable and can be used as a carbon free source
of combustion for CHP systems in the future.

Renewable Fuels

Renewable fuels such as biogas-landfill gas or digester
gas; biomass-waste wood or wood chips; and in the
future, synthetic natural gas and even hydrogen can fuel
CHP systems, Foley said.
When using renewable energy fuels, there can be fuel
quality and consistency of supply issues. Foley said these
challenges have been previously overcome by using
existing technology, so the real challenge is avoiding
poor planning.

Technical Challenges

For CHP to meet its efficiency, emissions reduction and
economic goals, it must be properly loaded electrically
and thermally, said Foley.
Sweetser said one of the first things he often sees when
he visits existing facilities interested in CHP is multiple
electricity feeds from the utility because of expansions that occurred over time at the site requiring more
power. In these cases, the facilities have to combine
those meters into a single point of coupling to create a
microgrid or CHP system. That process can be expensive, and sometimes limits a facility to a single feeder to
design for resilience, he said.
Understanding the electrical load is usually easy with
interval data from the utility, said Sweetser, but understanding the thermal load is the tricky part. Thermal
load quality, year-round thermal demand and thermal
system connectivity are challenging issues for the CHP
designer, said Foley. He said he tells engineers that if
they have 60 minutes to review a site, they should spend
55 minutes in the boiler plant, and do whatever else
needs to be done with the five remaining minutes. He
said the single biggest failure point for CHP projects with
or without renewable energy stem from thermal load
issues.
"You need to understand the seasonal variation
in those loads. And that's where the critical error is
often made," Foley said. "So, you've got to be able to

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ASHRAE Journal - December 2019

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