Figure 1.
(a) Scheme of the nZEB building with the solar collector, the low enthalpy geothermal subsystem and the dynamic thermal barrier; (b) Building prototype; (c) Thermal image of the nZEB South façade.
2.1. Thermal Solar Energy Collector
A thermal solar energy collector has been integrated in the metal roof of the building. The roof is composed of an external layer of corrugated steel, shaped as a square hip roof. Two polypropylene (PP) tubes circuits are fixed to the corrugated steel’s inner face. The thermal energy captured by the metal layer is transferred to a water-glycol fluid flowing in the pipes, supported by a thermal multilayer insulation blanket and a polystyrene insulation board (
Figure 2a).
The collector is responsible for the capture of the solar energy and plays a fundamental role in the energy balance of the building. A model based on the finite element simulation software Comsol Multiphysics [
27] has been developed to characterize the collector behaviour. It is divided in three zones that correspond to the different parts of the roof: the collector surface, where heat transfer processes between environment and collector occur; the pipes, where fluid flows, and the bottom insulation layer.
The thermal energy captured by the collector surface is transferred to the fluid, to be transported where it is demanded. Therefore, the energy balance in the collector layer determines the amount of solar energy that can be absorbed by the fluid, i.e., the solar energy available for use.
The thermal balance in the collector is shown in
Figure 2b and is given by Equation (1):
where G
rad(λ) and J
rad(λ) represent the energy gains and losses due to the radiation processes, respectively, depending both on the external and the collector temperatures, and G
conv is the thermal flow generated by convection.
Figure 2.
(a) Thermal solar collector scheme; (b) Energy balance in the collector surface.
Figure 2.
(a) Thermal solar collector scheme; (b) Energy balance in the collector surface.
Radiation gain is defined by the total incident radiation, short and long-wave. The short-wave radiation (SR) corresponds to the direct solar radiation, meanwhile long-wave radiation (LR) includes those produced by all bodies surrounding collector that emit any radiation.
According to Stefan-Boltzmann’s equation, the radiation gain is formulated as follows:
where
Tamb refers to the external temperature, and
αλl and
αλs (alpha values), represent the average absorptivity for the long and shortwave spectrums of the total incident solar radiation, including both beam and diffuse radiation;
σ is the Stefan-Boltzmann constant, and
f(Ω) is the view factor between long-wave emitter and the surface. For a quasi-horizontal inclination of the solar collector,
f(Ω) reaches a value close to zero. Thus, long-wave radiation has been considered negligible compared to the short wave one.
Radiation losses are given by Equation (3):
where
is the emissivity of the solar collector and
Tcol refers to its temperature.
Convective processes are described by Newton’s cooling law, Equation (4):
where h represents the convection coefficient, which is dependent on the type of media, gas or liquid, the flow properties such as velocity, viscosity and other flow and temperature dependent properties.
According to the energy balance, the total energy captured by the surface is presented in Equation (5):
This energy balance is partially controlled by means of the optical properties of the surface, as the energy captured can be optimized using a high shortwave absorptivity and low long-wave emissivity material. This behaviour is simplified applying the Kirchhoff’s law (emissivity equals to absorptivity), and considering the surface as a grey body (optical properties independent on the wavelength). Finally:
The boundary conditions of this external capture surface are obtained from Equation (6). A high value of the absorptivity optimizes the capture of energy when the solar radiation is higher than the radiation losses. That is, for certain given external conditions, the optical characterization of the collector is a relevant factor for its thermal performance.
The behaviour of the solar collector has been analysed carrying out a sensitivity analysis, comparing the average collector surface temperature for different values of absorptivity, 0.1, 0.4 and 0.7 (
Figure 3). The weather conditions are obtained with the TRaNsient SYstem Simulation (TRNSYS) software [
28] and correspond to a standard meteorological year in Madrid (Spain). The solar radiation in the collector is displayed in
Figure 3a. The simulation is carried out for a period of 10 winter days with a sampling of one hour. The results of the simulation are represented in
Figure 3b. The temperature within the yellow band corresponds to the comfort band, 20 °C to 25 °C. Only when the collector temperature is higher than 25 °C, the energy captured covers the heating demand.
Figure 3.
(a) Solar radiation in Madrid (Spain) in winter (10 days); (b) Temperature for different roof solar collector alpha values.
Figure 3.
(a) Solar radiation in Madrid (Spain) in winter (10 days); (b) Temperature for different roof solar collector alpha values.
These results show the great influence of the absorptivity on the solar collector performance. Despite being in winter in Madrid (low temperatures), the average roof temperature reaches values above the comfort band when the wall absorptivity is greater than 0.4. The higher the absorptivity, the higher the available useful energy. Therefore, it is possible to use the thermal energy captured by the roof for heating purposes if the external surface is provided with optimal optical properties.
The simulation also shows how the available energy for cooling, with temperatures lower than 20 °C, is independent of the surface absorptivity. Therefore, the selection of a dark colour for the roof improves the thermal energy capture for heating purposes, without having a great impact on the thermal energy for cooling. The thermal circuit of the building transfers this solar energy captured in the roof either to the ground heat exchanger (
Section 2.2), or to the dynamic envelope (
Section 2.3).
2.2. Ground Heat Exchanger: Low Enthalpy Horizontal Ground Thermal Energy Source
The ground storage system, made by concentric rings of polypropylene (PP) tubes, is located horizontally below the construction area (
Figure 4). The soil is divided in four zones, namely: HOT, WARM, COOL and COLD, thermally independent of each other. The building perimeter is limited by a 10 m square, where hot and warm zones are located in concentric squares within the inner 5 m, and the rest is used as a cool zone. The cold zone is outside the building perimeter. These divisions were built by using expanded polystyrene (EPS) sheets of 2.4 m × 1.2 m × 0.12 m. These walls were placed vertically, as shown in the
Figure 4, creating isolated areas to reduce the influence of weather conditions, such as the solar radiation, on the inner building [
29].
Figure 4.
Cross section of the ground heat exchanger.
Figure 4.
Cross section of the ground heat exchanger.
The temperature ranges for the different thermal zones are shown in
Table 1. The linguistic terms associated to these thermal zones of the ground heat exchanger will be used to control the thermal flow of the building.
Table 1.
Linguistic terms associated to temperature ranges for the different zones, and pipes characteristics.
Table 1.
Linguistic terms associated to temperature ranges for the different zones, and pipes characteristics.
Temperature | Range | Characteristics |
---|
HOT | 19–21 °C | 2 PP pipes, 0.22 m, L 200 m, located 3 m underground, buried below the building |
WARM | 17–19 °C | 2 PP pipes, 0.22 m, L 200 m, located 2 m underground, buried below the building |
COOL | 15–17 °C | 3 PP pipes, 0.22 m, L 200 m, located 2 m underground, buried below the building |
COLD | 12–15 °C | 2 PP pipes, 0.22 m, L 200 m, located 2m underground, buried outside the building |
To summarize, the ground, characterized by a high thermal inertia, is used as a heat storage tank that accumulates the thermal energy (heating/cooling) according to the fluid temperature [
30]. Absorbing/rejecting heat from/to the earth changes the soil temperature over years. This effect is shown in the results obtained in the experiments in the nZEB prototype. Nevertheless, the soil is used as a storage tank and do not consider this factor for multi-year operations. Thermal energy stored is used for heating/cooling the building envelope during cold/hot periods, on a diurnal or seasonal basis, and to keep the building in the comfort band along the year.
2.3. Dynamic Envelope: Thermal Barrier
The building envelope acts as a passive thermal barrier. This can be a key element in order to maintain a desired temperature in a building, particularly in cold climates, where the isolation layers of the walls can reduce the heating requirements. Besides, the right selection of the building envelope composition not only produces a better insulation but can also smooth the temperature peaks during the day-night cycle. In this work a dynamic thermal barrier with high thermal inertia is proposed. By using a controlled fluid flow in the inner zone of the exterior walls, the influence of the daily and seasonal outdoor temperature variations on the inner temperature of the building is reduced.
The wall is composed of three layers: two polystyrene insulation layers and, in the middle, a concrete layer. The parameters of the walls of the real nZEB building are listed in
Table 2.
Table 2.
Wall layers parameters.
Table 2.
Wall layers parameters.
Lightweight Concrete | Value | Units |
---|
Thickness | 0.15 | m |
Density | 1200 | kg/m3 |
Specific Heat | 800 | J/kg·K |
Thermal Conductivity | 0.57 | W/m·K |
POLYSTYRENE | | |
Thickness | 0.05 | m |
Density | 35 | kg/m3 |
Specific Heat | 1450 | J/kg·K |
Thermal Conductivity | 0.036 | W/m·K |
nZEB | | |
House Height | 5 | m |
House Width | 10 | m |
House Length | 10 | m |
Windows | 15 | |
Window Height | 1 | m |
Window Width | 1 | M |
Window Density | 2500 | kg/m3 |
Window Specific Heat | 840 | J/kg·K |
Window Thermal Cond. | 0.78 | W/m·K |
Initial Temperature | 7.5 | °C |
The wall presents a notorious thickness, low thermal conductivity, and high density and specific heat. The combination of these properties provides an extremely high thermal inertia to the walls.
PP tubes are embedded in the concrete layer, as shown in
Figure 5a. Thus, a fluid can flow through them to increase the thermal capacity of the concrete layer, and therefore the insulation, by controlling its temperature.
The thermal performance of the dynamic thermal barrier has been modelled and simulated with COMSOL Multiphysics,
Figure 5b, taking into account the energy transfer by conduction through the envelope. The initial values are: T_indoor = 22 °C and T_outdoor = 5 °C.
Figure 5c shows the temperature distribution (isothermal contours), when a warm fluid at 17 °C is injected in the PP tubes of the thermal barrier. The indoor-outdoor temperature gradient with two different fluid temperatures (warm = 17 °C and cold = 9 °C) is shown in
Figure 5d.
Figure 5.
(a) PP pipes between two insulation layers of the wall; (b) Simulation of the finite elements wall model; (c) Wall isothermal distribution; (d) Temperature vs thickness.
Figure 5.
(a) PP pipes between two insulation layers of the wall; (b) Simulation of the finite elements wall model; (c) Wall isothermal distribution; (d) Temperature vs thickness.
In the absence of PP tubes in the concrete layer, the temperature drop is due to the polystyrene layer because its thermal conductivity is less than 1/20 (0.028 W/m·K) compared to the thermal conductivity of the concrete wall. That is why the concrete wall is virtually isothermal at the indoor and outdoor average temperature. The pipes act as a temperature shield between the two insulation layers.
The temperature plateau of the dynamic thermal barrier for both warm and cold thermal energy flows represents the boundary condition. However, the temperature variation along the dynamic thermal barrier is low compared with the variation experienced along the two external layers. Therefore the thermal barrier reduces the thermal gradient and smooths the temperature variations. Thus, less thermal energy is required to reach an indoor comfort temperature.
A thermal energy model of the building, that includes the dynamic envelope, has been developed using the computational software Matlab/Simulink (
Figure 6a). The model represents the heat balance between building and environment through the envelope, including heat convection in both the inner and outer surfaces, and heat conduction through the envelope layers. These processes can be also represented by an equivalent circuit,
Figure 6b.
Figure 6.
(a) Thermal energy transfer model of the experimental building; (b) Thermal equivalent circuit.
Figure 6.
(a) Thermal energy transfer model of the experimental building; (b) Thermal equivalent circuit.
Newton’s law of cooling has been used (Equation (7)), to represent the energy transfer in a surface in contact with a fluid by the convection process. In current case the fluids are the inner and external air:
where h is the convection coefficient, which depends on external conditions such as wind velocity, humidity or air density, and surface characteristics like geometry or roughness. ΔT is the difference of temperature between the surface and the environment, and A the surface area.
The thermal resistance due to convection is defined in Equation (8):
The energy transfer through the envelope is calculated with the heat transfer law (Equation (9)), and represents the energy variation through the thickness of the wall combining thermal resistance and thermal capacity:
where k is the heat conductivity, C
p the heat capacity, and ρ the density of the material.
The thermal resistance due to conduction is formulated by Equation (10):
And the thermal capacity of each layer, Equation (11), is:
As the envelope layers are connected in series, the equivalent resistance is obtained by adding up the individual resistance of each layer, and the opposite with the inverse equivalent capacity.
The simulation results show the influence of the external conditions on the indoor temperature of the building. In
Figure 7, the indoor and wall temperatures obtained by the model are represented and compared with the measured external temperature, showing the influence of seasonal and daily weather variations, using the values displayed in
Table 2. The indoor temperature is clearly affected by external weather conditions, presenting the same behaviour than the outdoor temperature, except for the delay and cushioning effect produced by the thermal inertia of the envelope (heat capacity). The temperature measured by the sensor located within the wall follows the same pattern but, in this case, it seems to be more independent of the day-night weather variations. It is clearly shown that in the first five months of the year an extra thermal energy (heating) is needed to reach the comfort temperature (red band). On the contrary, in the summer an extra thermal energy (cooling) is required for the building to keep the desired temperature (blue band).
Figure 7.
Thermal inertia, seasonal and daily temperatures. Outdoor measured temperature, and both indoor and wall simulated temperatures.
Figure 7.
Thermal inertia, seasonal and daily temperatures. Outdoor measured temperature, and both indoor and wall simulated temperatures.
Besides, a sensitivity analysis was carried out, varying some of the wall parameters (
Figure 8). The thermal mass is defined by three characteristics: specific heat, density, and thermal conductivity. For instance, a thermal mass change of 40% represents a similar variation of each property. For this study, the parameters analysed are: the thermal mass (TM_concrete and TM_eps), the concrete thermal capacity (Cp_concrete), and the polystyrene thickness layers (T_eps). The values of the indoor temperature while changing these parameters are shown in
Figure 7. Remark that the thickness of the polystyrene layers (T_eps) is the most relevant parameter, when the wall is 90% thinner the indoor temperature increases around 20%. Similar behaviour is caused by changing the thermal mass of the concrete layer (TM_concrete), being the indoor temperature more affected by lower values. The variation of the temperature with polystyrene thermal mass (T_eps) confirms that the selected polystyrene parameters (
Table 2) were adequate, when this parameter increases the temperature goes up but slowly. Finally, lower concrete thermal capacity (Cp_concrete) increases the indoor temperature. In all these cases the model response is as expected. The thickness of the polystyrene layer does not strongly influence the indoor temperature, opposite to the polystyrene thermal mass behaviour. In addition, it is easy to infer that if the thickness of both the concrete wall and the polystyrene increase, the temperature variations will be reduced, although a much better insulated building also increases the risk of overheating [
31]. It is worth noting that Madrid is a city with a continental-Mediterranean climate, with average monthly temperatures of 5 °C in winter and 25 °C in summer.
Figure 8.
Effect of the building envelope parameters in the indoor temperature.
Figure 8.
Effect of the building envelope parameters in the indoor temperature.
To summarize, once the hydraulic circuit (
Section 2.4) is connected, the fluid flows through the PP tubes embedded into the exterior wall (dynamic thermal barrier) and different temperature ranges are selected from the ground heat exchanger and solar collector, according to user profile, time and season.