Solar Thermal Information

3
1. INTRODUCTION
Most Americans are increasingly aware of rising energy
prices and the environmental implications associated with
continued use of conventional fuels. “Sustainable living”
is one of the most prevalent topics being discussed in a
variety of media.
This situation has created growing interest in renewable
sources, such as sun, wind and biomass materials. It is
also fostering a rapidly expanding market for equipment
that harvests this energy.
Hydronics technology is the “glue” that holds most
thermally based renewable energy systems together.
Although heat sources such as solar collectors, solid-fuel
boilers and geothermal heat pumps are indispensable
components in such systems, they are not the sole
determinants of efficiency, energy yield or financial viability.
Without proper heat conveyance, even the best renewable
energy heat source will not perform as expected. Thus,
the proper application of modern hydronics technology
is vital to the continued growth of the thermally based
renewable energy systems market.
This issue of idronics focuses on systems that use solar
energy, as well as an auxiliary energy source, to supply a
portion of the domestic hot water and space heating needs
of a building. Such configurations are commonly called
solar thermal combisystems. Several design variations will
be introduced and discussed in the context of residential
and light commercial building applications. In each case,
state-of-the-art hydronic technology such as variable flow,
manifold-based distribution, hydraulic separation, thermal
mass and precisely controlled zoning are used to enhance
the system’s energy efficiency, enabling it to deliver the
same unsurpassed comfort, reliability and long life as that
provided by a well-designed conventional hydronic system.
Both piping and control aspects of solar combisystems
are illustrated and described. In many cases, the control
techniques are similar to those used in modern “non-
solar” hydronic systems. When properly applied, these
techniques allow combisystems to smoothly transition
between use of solar and auxiliary energy so that
occupants experience no difference in comfort.
The energy savings potential of such systems will also be
discussed. The goal is for designers to develop reasonable
expectations for what typical solar thermal combisystems
can provide based on differences in climate, system size
and loads.
2. FUNDAMENTALS OF
SOLAR COMBISYSTEM DESIGN
The largest sector of the solar thermal market is domestic
water heating. This is true both in North America and
worldwide. The underlying reason is the capacity to use
solar energy on a year-round basis, and in particular, the
ability to collect solar energy when it is most abundant — in
summer. Solar thermal systems for heating domestic water
were discussed in detail in idronics #3 (January 2008).
A natural extension of a solar domestic water heating
system is adding capability to offset a portion of the
space heating load in the same building, and hence the
name “combisystem.”
Most combisystems intended for residential applications
treat domestic water heating as the primary load, and
thus take advantage of high solar energy availability in
summer. Beyond their DHW “base load,” combisystems
typically use greater collector area and larger storage
tanks to capture and contain additional energy that can
offset a portion of the building’s space heating load.
As with solar DHW systems, combisystems require a
reliable means of freeze protection, as well as an auxiliary
energy device that supplies the energy required for
uninterrupted delivery of hot water and space heating
when solar heat gains cannot cover the load.
ESSENTIAL DESIGN PRINCIPLES:
The following design concepts are imperative to the
success of a solar combisystem. Each will be discussed
in the context of specific system designs described later
in this issue.
• The cooler the solar collectors can operate, the higher
their efficiency, and the greater the amount of solar
energy they harvest.
SOLAR THERMAL COMBISYSTEMS