Explicit Model for Solar Air Heaters Performance Assessment for

Winter and Summer Operation

CATHERINE BAXEVANOU*, DIMITRIOS FIDAROS, ARIS TSANGRASSOULIS

Department of Architecture, School of Engineering

University of Thessaly

Pedion Areos, 38334, Volos

GREECE

With remote heat storage system of phase change materials. For the cooling period the (b) configuration was

considered. The developed model is consisted by a set of equations that can be solved sequentially (no need for

software and/or programming) and can be in the early stage of building design to maximize the yearly utilizable

heat gains. The heat storage wall can increase the utilization of thermal gains from 15% to 46%. The use of

phase change materials can increase these thermal gains up to 77%. The successful summer operation depends

on the external climatic conditions and requires management of operation with an automation system when the

thermal solar gains overweight the ventilation losses.

Key-Words: - Passive solar systems, heat storage, quasi-steady explicit model, Trombe

wall, solar chimney, solar heating, passive buildings

whether they are new buildings or renovations of

existing buildings [1]. In addition, for the design of

new buildings, their energy efficiency, and therefore

the integration of passive systems in the shell,

should be taken into account from the initial stages

of their design [2]. Solar Air Heater is a quite simple

low maintenance device capable to increase fresh air

temperature and thus, it can be used in several

applications requiring low to moderate increase in

temperatures such as space heating, drying, etc. A

during the summer it can offer cooling by operating

as a solar chimney.

In its winter operation the wall stores heat and

supplies it to the adjoining room either by

its operation in the early stages of design.

Numerous software tools have been developed by

many researchers for the simulation of a Trombe

wall operation (EES, T*SOL, TRANSOL) [3,4,5].

The existing analytical models suffer from two

shortcomings: The first is that even in the case of a

monthly time step require iterations and thus some

kind of programming effort, the second is that they

require the knowledge of data like heat transfer

coefficients and dynamic parameters for which the

fact, the most widely used BES models are the

EnergyPlus [9, 10] and the TRNSYS [11, 12]. Yet

they require the purchase and learning of software,

so they are no adequate for early-stage design.

The next alternative is the use of CFD, which can be

used either for the simulation of the operation of a

whole Trombe wall along with the served room [9,

10, 13], either for the study of operation and design

of discrete parts of a Trombe wall [14]. The use of

CFD requires special knowledge, software, and

Holds only for mechanically ventilated Trombe

walls (requires the knowledge of airflow through the

air gap) and (ii) Does not account for the available

thermal mass. Since this model concerns the

operation of forced ventilated wall it cannot be

directly applied for naturally ventilated one. It

should be noted that the ISO 52016 [17], which

replaced the ISO 13790, does not address the

Trombe wall. In addition, the majority of these

models relate to the winter use of the Trombe wall

In the present paper, user-friendly, simplified and

explicit (they do not require iteration procedure for

the resolve of the model’s equation system, which

can be solved sequentially) quasi-steady models are

developed for winter and summer operation of a

Trombe wall, based on the concept supported by

room through Trombe wall.

between the glasses which can be taken from ISO

6946:2007 depending from the glass configuration.

Where, Z [W/m2K] is a parameter with which is

taken into account the temperature evolution of the

air along the air gap.

Where, RVR [m2K/W] is the star equivalent

resistance in the air layer calculated from the

Where, γal [-] is the gains/losses ratio of the air layer

calculated from the following relationship

 

Where, Qgn,sw [kWh] is the solar heat gains of the air

layer during the examined month and Qht,al [kWh]

heat losses of the air layer during examined month.

Where, Iw [kWh/m2] is the total monthly solar

energy incident on Trombe wall, FW [-] is the

internal heat sources which is taken zero since only

the operation of the Trombe wall is considered and

Qsol [kWh/mo] is the solar heat gains which is

calculated from the following relationship

Where, Φsol [W] is the time-average heat flow rate

from Trombe wall, brt,l [-] is the adjustment factor

for the adjacent unconditioned space, Φsol,l [W] is

the time-average heat flow rate from solar heat

source l in the adjacent unconditioned space which

is taken equal to zero since only the Trombe wall is

Where, Fsh [-], is the shading reduction factor for

external obstacles, Asol [m2] is the effective

wall solar gains are utilized as a whole. We know

that in practice this does not happen. So, the above-

developed model describes an ideal operation with

which we can calculate the maximum limit of the

Trombe wall performance. In practice, the wall

Trombe performance depends on the available heat

capacity of the conditioned room plus the passive

element’s heat capacity. The influence of available

heat capacity is modeled through the calculation of

the gain utilization factor nH,gn. According to the

(34)



Where, αΗ,0, a dimensionless reference numerical

parameter, equal to 1 for monthly calculation

method, τΗ,0, a reference time constant equal to 15

[h] for monthly method.

Where, Cm, [J/K] the internal heat capacity of the

building zone, Htr,adj, [W/K] the overall heat transfer

without thermal mass for heat storage, Cm=0

and αH =1

appropriate mass thermal storage wall Cm =

the storage wall heat capacity

storage system away from the opening for day–

night operation. Cm = storage wall heat

capacity + remote storage system heat capacity.

In the case in which the remote storage system

upper section of the transparent cover is open to the

environment. Additionally, an open window in the

north wall allows external air enters to the room.

In Figure 1 a cross-section of the examined

geometry without pcm (cases a and b) is given while

in Figure 2 the cross-section with pcm is presented

(case c). In Figure 3 the cross-section of the solar

chimney operation is presented.

In Figure 4 the monthly average flow rate though

In Figure 5 the monthly average flow rate through

the air-gap according to [19] for the summer

operation is presented.

Figure 5. Mass flow rate through the air gap

3 Results and discussion

3.1 Results and discussion for the heating

period

storage mass. It comes out that the influence of

thermal storage mass is insignificant. The ideal heat

In Figure 9 are presented: a) the solar gains from

Trombe wall, QC,gn [kWh/mo], which correspond

to the cooling loads attributed to the operation of the

wall, and b) the heat losses (through the shell and

ventilation) due to wall operation as a solar

chimney, which corresponds to the wall contribution

to the cooling for the examined summer months.

Figure 11 shows a comparison of the heating gains

from the Trombe wall, QC,gn [kWh/ mo], during the

examined months, which correspond to the

contribution of the wall to the cooling loads. The

comparison shows very small differences with a

slight reduction of the burden during the months of

Figure 12 comparatively shows the losses (shell and

ventilation) from the Trombe wall corresponding to

the cooling contribution, QC,ht [kWh/mo] for the

summer months under consideration. Losses appear

to be more affected by both radiation and

temperature compared to the gains. Thessaloniki

almost identical to case0. However, as the

temperatures decrease, the contribution to cooling

increases (Ioannina), while in Kastoria the

contribution remains positive even during the

months of July and August.

Figure 12. Heat losses through Trombe wall in the

examined locations