• Energy Research

AbstractThe CO2-neutral self-supply of heat and electric energy is an important objective for new and existing buildings in the future [1,2]. Therefor the energy autonomous house (EAH) as a new concept for single-family buildings in central Europe is presented. It represents a further development of the solar and efficiency house concepts based on full self-sufficiency in thermal (partly provided by a fireplace) and electrical energy (100%). Two occupied houses have been built in Germany and they are under an extensive scientific monitoring with real user behavior since 2014. This contribution is focused on thermal energy balances and the differences due to different user behavior and the influence of weather conditions. The evaluated solar fraction was fsol, th ≥ 71.4% and fsol, el ≥ 91.8% for both houses in 2014. So far the 100% autonomy in electricity could not be reached due to the unusual low irradiation in Jan. and Dec. 2014 (-24% / -37% compared to long term values). Nevertheless the planned low electricity consumption of ∼ 2000 kWh/year could nearly be achieved, whereby a self-consumption rate of electric energy gains of ≥ 31.8% were assumed. Further findings of 1 ½ years of monitoring of the two EAH are presented within the paper.

A survey is given on condensation heat transfer inside a two-phase thermosyphon including a detailed description of fluid flow as well as a review of published experimental and theoretical investigations. A total of 2889 data points derived from 18 research works has been evaluated covering wide ranges of thermal and geometrical parameters: 10 different working fluids, saturation temperature (14°C ≤ ts ≤ 340°C), pressure (0.04 ≤ p ≤ 39.5 bar), inside tube diameter (14 ≤ d ≤ 66 mm), length of cooling zone (102 ≤ L ≤ 2450 mm), and inclination angle (0° ≤ ϑ ≤ 85°). Correlations are proposed for condensation heat transfer inside a two-phase thermosyphon. Remaining deviations are due to effects of shear stress at the liquid-vapour interface, effects of non-condensable gases and effects of a partial flooding of the cooling zone in the case of superfilling the thermosyphon.

Abstract Experimental investigations with four large thermosyphons identical in construction operated with titanium dioxide and gold nanofluids and deionized-water as reference fluid are carried out. It is experimentally shown that the thermal resistance can be lowered by employing nanofluids only at small values of heat flow rates provided at the evaporator. The best results are achieved with a gold nanofluid at heat flow rates between 50 W and 150 W. The lowering of the thermal resistance is here about 20%. Accompanying distillation experiments clearly indicate that only some per mill of the nanoparticles originally suspended in the nanofluid are torn out of the working fluid and transported by the vapour phase. Beside descriptive explanations and diagrammatical representations of the experimental results three videos are provided. These videos allow for the first time in situ observations of the thermosyphons interior when operated with nanofluids in real time.

Abstract An experimental investigation was performed in order to observe the effect of the inclination angle on the transport behaviour of a closed two-phase thermosyphon. A vertical or inclined steel pipe was used; this was electrically heated at the lower part (‘heating zone’), and it was cooled by water at constant temperature along the upper part (‘cooling zone’). Between these two zones there was the well insulated ‘transport zone’. As working fluid refrigerant R115 was used. The quantity of fluid in the tube was taken so that the critical specfic volume v crit was obtained. Thus, a mean thermodynamical critical state can be reached. The heating zone was always flooded. The electric heat input and the inclination angle ϕ towards the vertical position were varied in steps. The maximum heat flow rate in the tube to depend upon the inclination, with a largest value of Q max = 2850 W at ϕ = 40° . An effective thermal conductivity λ eff of the thermosyphon can be determined depending strongly on inclination and heat flow rate; roughly speaking, the steeper the tube and the larger the heat flow rate, the higher is the effective conductivity. Seperate consideration of the transport regimes in the various zones showed their individual influence upon the overall behaviour. The condensation in the cooling zone exhibits the largest transport resistance. The heat transfer with boiling in the heating zone shows large local differences and depends strongly on inclination.

A new model MISOS is proposed for the simulation of the borehole filling (grout) of double U-pipe heat exchangers. When simulating ground-coupled heat pumps, a suitable model of the filling is necessary because the temperature of the filling effects the temperature of the heat carrier fluid. The filling is divided into three elements whose geometry corresponds to the different temperature zones. For each time step, the temperatures of the filling elements can be calculated from energy balances. MISOS is very fast compared to computational fluid dynamics (CFD) algorithms. CFD calculations were performed for different shank spacings, and results compared with those obtained from MISOS. If the pipe shanks are situated between the axis and the wall of the borehole, nearly the same difference of the fluid temperature between inlet and outlet is predicted by MISOS and CFD. For a minimal shank spacing, heating is overpredicted by about 6% for an extraction period of 3 h while an underprediction of about 9% is obtained for maximal shank spacing.