Storing Summer Heat
for Winter
28
January/February 2011
SOLAR TODA Y
solartoday.org Copyright © 2011 by the American Solar Energy Society Inc. All rights reserved.
B
efore electric appliances came on the scene, we cut ice from frozen lakes and rivers and buried it to use for refrigera-
tion long after the rivers had melted. What if
there was a way to do the reverse, storing sum-
mer heat for warmth during the winter?
Space heating comprises nearly half of the
energy used in a typical U.S. home. With retire-
ment in mind, I became interested in high-mass
solar heating — storing heat from solar thermal
collectors in water or sand — as a means of reduc-
ing our monthly utility bills. A look at annual heat-
ing demand and solar energy supply overlaid in a
single graph further underscores
the need for effective long-term
solar thermal storage (figure 1).
I’ve also gotten interested in ener-
gy-efficient housing, in hopes that
my grandchildren can avoid ever-
increasing utility bills when they
have their own homes.
Our house is like many built
in Virginia, a one-story ranch
built on a crawl space. I originally
designed our home’s solar energy
system to provide both radiant
space heating and domestic water
heating using water tanks. I tried several varia-
tions to optimize space heating, but it seemed
impossible to design a system big enough to rea-
sonably heat the house without generating exces-
sive hot water during the summer.
I first learned about high-mass solar heating
while attending a solar hot water training pre-
sented by Bob Ramlow at the Midwest Renew-
able Energy Association (MREA, the-mrea.
org). I later saw Bob’s own system at his net-
zero-energy home in Amherst, Wis. It was simple
and it worked. Through a gridwork of radiant
tubing, collected heat is delivered to and stored
in a sand bed beneath the house; heat is pro-
vided through the home’s floor by conduction.
(See Ramlow’s
SOLAR TODAY
article, “Solar-
Heat Your Home,” Nov./Dec. 2009: solartoday.
org/ramlow.) But not a lot was known about
the design of the high-mass sand bed; we know
what was done but not why it works or how to
optimize it.
Mechanical engineering colleagues at Vir-
ginia Commonwealth University joined me in
solving this challenge. Associate Professor James
T. McLeskey Jr. and master’s candidate Marshall
L. Sweet helped develop a system to retain sum-
mer’s peak heat for solar thermal heating during
the cold months. As a bonus, the storage system
uses conventional building materials and tech-
niques, making it easy for a builder to do.
designing the virginia heatstore
Most solar thermal heat-storage systems to
date have used large water tanks. Large water
tanks are expensive and structurally complex.
High-mass systems avoid these problems, but
they cannot be “turned off.” Once heated by the
solar collectors, the sand bed transfers its heat to
the building’s floor until the heat is gone. Inside
temperatures can reach as high as 86°F ( 30°C).
The Virginia Heatstore was designed to take
advantage of sand bed design but allows the user
to store heat year-round in a sort of solar oven. We
are researching the size, shape and construction of
the heatstore. As shown in figure 2, the sand bed
is isolated from the floor; it can even be located
away from the house. Heated water from the solar
panels is carried through PEX tubing in the bed,
We simulated the performance of an under-
ground heatstore for a single-story ranch-style
home in Richmond, Va. To do so, we used
software called TRNSYS (“tran-sis”), or TRaN-
sient SYstem Simulation program, designed
to simulate the transient performance of ther-
mal energy systems. The TRNSYS component
library enables users to simulate complex energy
systems by selecting system components and
linking their inputs and outputs.
As shown in figure 3, our simulation used
many components from the standard TRNSYS
component library. The major components we
used were Type 56 (Building), Type 701 (Base-
ment) and Type 76 (Theoretical Flat Plate Col-
lector). The heatstore bed itself was modeled
using Type 342 (Multi-Flow Stratified Thermal
Storage Model with Full-Mixed Layers), which
we purchased from Transsolar Energietechnik,
an engineering firm. Type 56 (Building) was
Figure 1:
Annual Space-Heating Demand vs. Useful Solar Energy
heating the sand. During the winter, a second set
of PEX tubing removes heat from the bed to heat
the house via a radiant floor. The water carrying
heat from the bed to the house is activated and
deactivated as needed by thermostats.
Simulating Performance
Modifying your home to include high-mass
heat storage can be expensive and risky. In addi-
tion, building inspectors are reluctant to permit
systems with which they are not familiar. In order
to size and optimize the Virginia Heatstore, we
needed to model the system.
Figure 2:
The Virginia Heatstore
Cover
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