The objective of the investigation is to evaluate the
potential of a direct solar integration into the space heating
circuit. Integrated into a full system layout such direct systems
are analyzed within the simulation environment
TRNSYS 17 (Klein et al., 2012). As a reference, also typical
combisystems with only one buffer tank are considered
(named ‘‘buffer system”). Both systems may be equipped
with different space heating elements – radiators, floor
heating or thermal activation of concrete elements. The
investigation also includes a system with a combination
of two types of heating elements, a thermal activation
directly heated by the solar thermal collector and radiators
solely heated by the auxiliary heater via the buffer storage
(named TA + Rad). All systems investigated are shown in
Fig. 1.
2.1. Space heating elements
The space heating elements play an important role for
the evaluation of the direct solar space heating and differ
in heat transfer rate, design temperature and storage capacity.
The three heating elements considered in the investigation
are shown in Table 1 with the main characteristics if
installed in the two-story building used in the simulations.
While radiators are heat emitters placed in the heated
zones of the building, thermally activated building systems
(TABS) are integrated in the building structure itself. A
very common form of TABS is a floor heating system
(FH) where water pipes are embedded in the upper layer
of the floor construction in contrast to a thermal activation
(TA). Here, the piping is installed deeper in the floor e.g. in
the middle of the concrete.
The large surface area of FH and TA allows a signifi-
cantly lower operation temperature level compared to
radiators which affects both the solar yield and the performance
of the auxiliary heater. Furthermore, the high
capacity of the building structure leads, especially in the
case of TA, to a slow response of the heating system to
abrupt changes in the internal loads or the passive solar
heat gains. The deeper the piping is installed in the
concrete, the slower is the response of the heating system
leading to higher floating in the room temperature