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Light pipes transmit daylight into building interiors. Their installation into thermally insulated roofs of low energy buildings can be a problem because of thermal bridges and condensation problems. This article is focused on a CFD simulation thermal analysis that includes four variations of light pipes with a segment of a flat roof. Common light pipes with a hollow light guiding tube were compared to special light pipes containing an additional glass unit located inside the tube. The additional glass units increase thermal resistance and reduce condensation risks of the light guiding systems. The light pipes were compared in two different simulation models run in ANSYS Fluent software and the CalA program. Temperature profiles and air flow patterns of the cross sectional profiles of the light pipes served to determine the total heat transmittance and heat losses of the studied light pipes installed in a segment of a thermally insulated flat roof. The paper compares simplified 2D rotational–symmetrical numerical model based on the thermal diffusion equation with the complex 3D CFD numerical simulation. The results confirm that the simplified 2D numerical model is suitable for the thermal evaluation of the light pipes containing an additional glass unit, too. The additional glass unit with the triple glass improves thermal resistance up to 88% in case of light pipe with diameter 600 mm and reduces optical transmittance to 28%. [ABSTRACT FROM AUTHOR]

• An alternative indicator of thermal response of radiant systems is studied (HSE). • HSE allows comparing systems with complex thermal behaviour by a single value. • HSE ranged from 3% for fast systems to 56% for slow systems. • Effective thermal output (ETO) that considers the thermal response is introduced. • ETO was up to 57% lower than nominal thermal output for thermally active systems. An alternative indicator of thermal response of radiant heating and cooling systems called heat storage efficiency (HSE) has been tested. The heat storage efficiency was compared with established indicators represented by the time constant τ 63 , response time τ 95 , and thermal energy stored (TES). The comparison was performed for three wall cooling systems with various combinations of pipe location, configuration of material layers, and materials of the thermal core. Considering the whole response curve (HSE) instead of focusing on one specific point on the curve (τ 63 , τ 95) allowed comparing the thermal response of complex systems by a single value. It also permitted predicting thermal response consistently regardless of the system and core material. Thermal energy stored predicted the thermal response of certain systems, but it may not be suitable for comparing the thermal response of radiant systems with different thermal admittance. Besides, a novel indicator of thermal output called effective thermal output (ETO) has been proposed. The effective thermal output considers both the steady-state thermal performance represented by the nominal thermal output and dynamic thermal performance represented by the heat storage efficiency. For thermally active building systems (TABS), effective thermal output was up to 57% lower than the nominal thermal output. [ABSTRACT FROM AUTHOR]

Numerical simulations and experiments were performed for a thermally active wall with pipes arranged in milled channels in the thermal insulation. The advantage of this system is its suitability for installation in both new and existing buildings in the form of precast heat insulation panels attached to their facades. The study shows that by active control of the supply water temperature, it is possible to alternate the walĺs function between space heating and a thermal barrier. The wall system has the potential to significantly reduce heat loss when used as a thermal barrier. When operated as space heating, embedding the pipes in thermal insulation reduced the heating capacity by 50% as compared to systems with pipes arranged in a concrete core and by 63% for pipes arranged in a layer underneath the surface. It is crucial that pipes arranged in channels are embedded in a thermally conductive material. Failing to do so can substantially diminish the heating capacity due to the imperfect contact between the pipes and radiant surface and also due to the air gap that may form around the pipes. The thickness of the thermal insulation, spacing of the pipes, and supply water temperature also have a substantial effect on the heating capacity, whereas the thickness of the concrete core does not. [ABSTRACT FROM AUTHOR]

[Display omitted] • Analyses are conducted focused on climate adaptive reactions of BiPV facade system. • System's performance prediction is relevant for energy-efficient buildings. • Optimisation techniques yielded an increase in total thermal and energy performance. Ventilated building-integrated photovoltaic (BiPV)/phase-change material (PCM) façades have been applied and validated in building energy simulations; however, the dynamic thermal response of these façades has not been investigated. Notably, performance predictions and simulations for systems featuring natural airflows in the façade cavity are important for guiding the decision-making for energy-efficient buildings. To address this challenge in literature, in this work, numerical analyses were conducted, focusing on the climate adaptive reactions of a BiPV façade system coupled with a latent thermal energy storage system, based on a PCM. Numerical methods for determining the PCM heat transfer were evaluated, including their limitations. The thermodynamic reactions of two BiPV façade concepts were comparatively studied using two simulation domains: building energy simulations and computational fluid dynamics. The reliability of the theoretical methods was also evaluated. Good agreement between the simulation results and experimental data was noted through dynamic outdoor tests, empirically validating the study; standard statistical indicators were calculated and employed to assess the consistency between the experimental and simulation results. The used numerical approach can reliably predict the thermo-responsive capabilities of PCM-based BiPV façades with respect to the overall tendencies. The parameter variation techniques revealed modifications in the overall thermal and energy performance of the façade system. The most undesirable instance of overheating was predicted when using RT27; therefore, the PCM is considered inappropriate in this case. [ABSTRACT FROM AUTHOR]

• Construction rules were studied for wall and ceiling systems suitable for retrofit. • Adding insulation was not necessary with insulating core even for low wall thickness. • Increase in output per 1 cm of pipe spacing was maximal at 6 cm. • A dense spacing maximized the system output per energy input. • A metal fin attached to a pipe in insulation led to the highest output. The application of radiant heating and cooling systems in building retrofit could facilitate the use of renewable energy sources in existing buildings. This research focused on adapting the design of a ceiling and wall system with pipes underneath the surface and a wall system with the pipe embedded in a bricklayer. These systems are suitable for installation in retrofitted rooms, but the findings are also applicable to new buildings. Heat transfer was computed using a validated numerical model. With a conductive core, insulation thickness of up to 3 cm was appropriate for an internal wall. With an insulating core, insulation was not necessary even if the core was only 15 cm thin. The increase in output per 1 cm of pipe spacing was maximal at 6 cm. Spacing below 3 cm was inefficient. A dense spacing maximized the system output per energy input by creating a uniform surface temperature, while also shortening the response time. Attaching a metal fin to a pipe in plaster increased the output by as much as reducing the pipe spacing from 10 to 6 cm (14 W/m2, i.e. 20%). Attaching a metal fin to a pipe embedded in insulation for a compact design led to the highest output of all cases studied. Placing the pipe in a brick layer added to a conductive wall did not increase the heat storage capacity of the wall. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water-based wall systems for heating and cooling and thermal barriers are reviewed. • A classification scheme is proposed for heating/cooling systems and thermal barriers. • Benefits and drawbacks are summarized, design recommendations are provided. • Alternating between heating, cooling, and thermal barrier is perspective operation strategy. • Incorporating PCM in the wall and use in building retrofit are research opportunities. This study reviews water-based wall systems for space heating and cooling and thermal barriers (TB) for the reduction of buildings' thermal load. The review gives a general overview of the research and groups it into subtopics that are discussed in detail. For space heating and cooling, the subtopics entail thermal performance, thermal comfort, renewable energy sources, use for building retrofit, and combination with phase change materials (PCM). For TB, especially the working principle, types and designs, and performance are discussed. A classification system is proposed separately for wall heating and cooling systems and TB based on the designs found in scientific literature. Benefits and drawbacks are summarized, and design recommendations are provided for the wall systems. It was shown that in certain cases, radiant wall systems can be preferable to radiant floors and ceilings, but further comparisons would be useful to provide conclusive evidence. For TB, the studies uniformly declare that TB reduce buildings' thermal loads and energy demands. Few studies focused on the economic and environmental aspects of using TB. Most of the studies about TB are based on calculations. Measurements to quantify the benefits of TB under real operation and refine the conditions under which various types of TB are feasible are lacking. Enhancing the wall performance by PCM in the active layer, application of the wall systems in building retrofit, and alternating between the functions of heating, cooling, and TB present the biggest research opportunities and challenges. [ABSTRACT FROM AUTHOR]

A radiant wall heating and cooling system with pipes attached to thermally insulating bricks was tested using climate chambers and a hotbox. This system is especially suitable for building retrofit due to its affordability and ease of installation but can be also applied in new buildings. Besides walls, the design tested can be also used for ceilings. Thermal output and response, wall surface and cross-section temperature, and water temperature were measured under a range of thermal loads. The thermal response was fast despite the coupling of the pipe with the bricks; the time constant τ 63 was 0.5 h. The low-conductivity core substantially reduced thermal losses meaning that the system can properly function even without thermal insulation. These qualities may present an advantage compared to systems with pipes coupled to a conductive core which require insulation and have longer response times. The difference between water and average surface temperature was small, up to 7.0 °C at the peak output of 100 W/m2, which benefits the energy source efficiency. However, the surface temperature was non-uniform, which should be considered to prevent local condensation. Numerical simulations at room level showed that locating the system at one wall leads to a non-homogeneous thermal environment. Installation at multiple walls can be preferable to attain more uniform conditions. [ABSTRACT FROM AUTHOR]