An ongoing new project
at the DOE’s National Renewable Energy Lab (NREL) aims to improve the
efficiency of HVAC systems by incorporating thermochemical materials into the
design. Salt hydrates are the most promising TCM for HVAC applications. According
to NREL:
“The TCM is discharged and charged through hydration and
dehydration reactions, respectively. Hydrating the salt releases heat, which is
used to heat the building, and extra heat from the heat pump at other times of
day is required to dehydrate, or charge, the TCM. This means the reactor needs
to interact with water vapor. This water vapor could come directly from the
ambient air, in which case the TCM is an open system. Or the TCM could be in an
isolated chamber, evacuated of air, which is known as a closed system. In this
case, the water vapor comes from evaporating liquid water from a second chamber.”
A February 2020
paper in the Journal of Energy Storage explored TCM storage via salt hydrates. The
authors concluded that the tech is promising. They also note that stability testing
should be done for a large number of cycles, system designs should be tweaked
to improve performance, and the use of moving and fluidized beds should be
explored. The authors also noted at the time:
“There are several technical challenges to the design of
an efficient and stable system which need to be addressed before
commercialization.”
Salt hydrates are
desirable due to their high energy density (400-870 kWh m−3) and low turning
temperature (<150 °C). The graphic below from the paper shows the hydration
and dehydration reactions.
Thermochemical energy
storage (TCES) utilizes a reversible chemical reaction: AB + heat = A + B.
“In the charging (dehydration) step, thermal energy is
used to dissociate the chemical bonds between molecules through an endothermic
reaction. The dissociated materials are then kept separately (storage step).
The stored …”
I assume the hydration step re-associates the
materials as indicated in the graphic. Self-separation of reactants and the use
of water vapor give salt hydrates an advantage as a TCM.
“Composite materials have been widely investigated to
minimize the limitations that occur when salt hydrates are used. This can be
done by using either mixture of material or through impregnation and
consolidation of salt into an inert (expanded graphite, vermiculite, etc.) or
active (zeolite, silica gel) material. The host matrix is important in order to
prevent salt agglomeration, and swelling, which leads to an improvement in
moisture diffusion during heat regeneration”
“Several factors regarding the material; kinetics, mass
and heat transfer, economic cost and safety should be taken into account when
designing a reactor.”
A June 2022 paper
in Applied Energy explored using Strontium chloride (SrCl2)-cement and
zeolite-13X materials in a cascade system.
“A cascade system consists of two independently operated
single-stage refrigeration systems: a lower system that maintains a lower
evaporating temperature and produces a refrigeration effect and a higher system
that operates at a higher evaporating temperature.”
The paper concluded that a cascade system with composite
materials had several advantages to a single material system:
• The cascade
system achieved high energy densities from 108–138 kWh m−3 over the dehydration
temperatures of 50–130 °C.
• The cascade
system improved on the exergy efficiency of the SrCl2-cement system by 6–38%.
• A cascaded
thermochemical energy storage system improves power output, temperature lift
and exergy efficiency.
Strontium
chloride gives off heat as it reacts with water vapor in the air. The recent NREL
research recounted in a December 2024 paper in Applied Energy explored several
configurations in different buildings and building types and paid special
attention to the source of water vapor. The study utilized computer modeling. The
study considered open-cycle TCM reactor configurations. According to NREL:
“The configuration with the best results allowed the TCM
reactor to heat the air exiting the building, which is at the same temperature
and humidity as the indoor air. Once heated, the air then indirectly heats the
incoming ventilation via a heat exchanger. This prevents the reactor from
dehumidifying the indoor air and provides a sufficient humidity level. In
addition to offsetting the energy required to heat the necessary ventilation
air, the air can be heated above the indoor temperature, reducing the energy
required by a furnace or heat pump to maintain the indoor temperature.”
This can overcome the challenge of preventing overly
dehumidifying the indoor space. However, only works for buildings that have the
exhaust air vent located near the incoming ventilation. The configuration was
found to work better in a winter with warmer and wetter air, as in Seattle, and
worst in a winter with cold and dry air, as in Minneapolis. Moist air is
required to drive the TCM reaction. NREL also noted:
“The low LCOS {less than 10 cents per kilowatt-hour} indicates
the technology has a feasible path to commercialization, but additional work is
needed to quantify the reactor manufacturing, integration, packaging, and
installation costs. Making this a cost-effective technology will require
addressing each of these costs. The researchers are also exploring other
options for integrating TCMs into HVAC systems, including the closed-cycle
systems mentioned above. These systems are not constrained by ambient humidity
but come with a separate set of challenges they hope to solve with further
research.”
References:
Scientists
make critical breakthrough with energy-storing chemicals — here's how it could
transform home heating. Rick Kazmer. The
Cool Down. December 5, 2024. Scientists make critical breakthrough
with energy-storing chemicals — here's how it could transform home
heating
News
Release: Thermochemical Tech Shows Promising Path for Building Heat. U.S. Dept.
of Energy NREL. November 13, 2024. News Release: Thermochemical Tech
Shows Promising Path for Building Heat | News | NREL
Open-cycle
thermochemical energy storage for building space heating: Practical system
configurations and effective energy density. Yi Zeng, Ruby-Jean Clark, Yana
Galazutdinova, Adewale Odukomaiya, Said Al-Hallaj, Mohammed Farid, Sumanjeet
Kaur, and Jason Woods. Applied Energy. Volume 376, Part A, 15 December 2024. Open-cycle thermochemical energy
storage for building space heating: Practical system configurations and
effective energy density - ScienceDirect
Experimental
investigation into cascade thermochemical energy storage system using
SrCl2-cement and zeolite-13X materials. Ruby-Jean Clark and Mohammed Farid. Applied
Energy. Volume 316, 15 June 2022, 119145. Experimental investigation into
cascade thermochemical energy storage system using SrCl2-cement and zeolite-13X
materials - ScienceDirect
State
of the art on salt hydrate thermochemical energy storage systems for use in
building applications. Ruby-Jean Clark, Abbas Mehrabadi, and Mohammed Farid. Journal
of Energy Storage. Volume 27, February 2020, 101145. State of the art on salt hydrate
thermochemical energy storage systems for use in building applications -
ScienceDirect
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