**2. Technical description of the thermoelectric heating units**

Since 2009, the authors have been working on alternative HVAC systems for buildings. Based on their previous experience as architects (not engineers) in the area of building services and energy systems [38], the authors have focused on the design of decentralised ventilated heating system for new and rehabilitated building envelopes. The result was the construction of a simplified inhabited housing unit (prefabricated module) with a thermoelectric heating system. A detailed report on this first THU version (v1.1) and its manufacture has been presented by the authors in previous works [33–36]. This section provides a brief description of this first version and presents the improvement of this prototype (THU v1.2). Both prototypes were installed in a prefabricated test room to analyse their performance under real conditions.

#### **2.1. Thermoelectric system of THU version 1.1**

The studies related to the cooling mode have focused their efforts on developing prototypes [6–12] that dissipate the heat in small applications such as in lasers, personal computers, refrigerators, cryogenic prototypes, and so on, while heating modes are being applied mainly in architectural area. The essential concepts of Peltier modules applied in the architecture have been introduced by Khire et al. [13], who proposed an ABE system that uses solar energy to compensate for passive heat losses or gains in a building envelope. The authors discussed the design and optimization of Peltier modules with PV panels in their work. Under this context, Xu et al. [14] developed various ABE prototypes in a heating mode, using commercially available PVs and Peltier cells, and Liu et al. [15] designed an ASTRW system with thermoelectric technology and PV panels. Likewise, Vázquez et al. [16] described the basic principles of a new concept for an active thermal wall that improves the current practice for designing and installing air-conditioning for enclosed spaces. Irshad et al. [17] designed a solar TE-AD system that employs thermoelectric modules (TEMs) inside an air duct to provide thermal comfort. In [18], a solar thermoelectric cooled ceiling is combined with a displacement ventilation system. The prototype was tested in cooling and heating modes. Other interesting studies [19, 20] described the application of Peltier cells in active walls, active building windows and thermoelectric ventilators. Recently, Luo et al. [21–23] proposed a building integrated photovoltaic thermoelectric wall system, which is supported by the co-work of PV module for solar radiation transformation, air gap for thermal dissipation and thermoelectric radiant panel system for active radiant cooling/heating. This study focused on an efficient and accurate system model for the simulation of this system. Wang et al. [24] develop a thermoelectric heating system powered by renewable energy to reduce

emission in buildings. According to results, the prototype minimises the energy

emissions.

The studies related to the economic analysis of HVAC technologies emphasise energy saving in the heating/cooling system [25–27] and the energy demand in buildings [28, 29]. From the engineering point of view, the number of Peltier cells, the heat exchanger design and the auxiliary system (fans, back-up system, control system, etc.) are studied in order to reduce the investment, operational and maintenance costs [30–32]. The purpose of this study is to present the conceptual design of a THU integrated into ventilated façade and analyse its economic viability and its thermal performance. To accomplish this aim, the following items

**1.** describe the conceptual design and the operating mode of an innovative THU prototype;

**2.** compare the thermal performance of a THU prototype with a conventional air-conditioning system using the typical economic indicators (investment costs, maintenance costs and op-

This work contributes in identifying the key aspect that may increase the efficiency of the HVAC systems in ventilated façades. Also, this research completes the authors' previous work [33–37] about the theoretical design and construction of an active ventilated façade with Peltier modules. The authors are aware that a techno-economic analysis of a THU prototype

the CO2

124 HVAC System

are proposed:

erational costs).

has not yet been reported.

demands and therefore reduces CO2

Thermoelectric heating unit (THU v1.1) consists of three subsystems: a heating system, a ventilation system and a control system. The heating system was composed of 84 RC12–8 Peltier modules (Marlow Industries, Inc.) with a heat dissipation system. The Peltier modules were placed in groups of two thermoelectric modules, where the Peltier modules were connected in series and the groups were connected in parallel; altogether, they form 42 groups that require a voltage of 50 volts and have a heating capacity of 3 kW (3/4TR heating tonnage). The elements of heat dissipation system are composed of 84 heat pipes, 21 finned heat sinks, two axial fans (fixed on the façade) and two tangential fans (fixed on the internal chamber). A schematic diagram of THU v1.1 prototype is illustrated in **Figure 1**.

The prototype has a control system that supplies electric energy to the system, controls the auxiliary equipment (fans, sensors, actuators, etc.) and regulates the working operations (inside temperature). In addition, a protection equipment was included in case of accidents that basically consists of a PLC, sensors and actuators.

**Figure 1.** Schematic diagram of THU v1.1: (a) parts of the prototype, (b) heating system (heat pipes with Peltier cells) and (c) external view showing the electrical connection (down) and the ventilation grills (sideways).
