# Experimental and Numerical Study of Thermal Performance of an Innovative Waste Heat Recovery System
## 1 Introduction  
According to a study [1] made in 2019, the building sector is responsible for about 16% of the wasted heat energy. The waste heat energy with a value lower than 100 \({}^{\circ}\)C is considered low-temperature, the one with a temperature between 100 \({}^{\circ}\)C and 300 \({}^{\circ}\)C is medium-temperature, while the waste heat energy with a temperature higher than 300 \({}^{\circ}\)C is considered high-temperature. The highest losses of thermal energy are in the low-temperature sector, with a percentage of 64%. Waste heat recovery can have a significant impact on the energy efficiency of buildings by reducing the heat losses. There are various heat recovery technologies such as waste heat boilers [2], economisers [3], plate heat exchangers [4], heat pipe heat exchangers [5] and others, which all have the purpose to capture the waste thermal energy and convert it to active thermal or electrical energy [6]. The waste heat utilization is very diverse and it has been developed in the last few years. There are numerous works in the literature which concentrate on waste heat recovery using heat pipes. The original contribution of this work consists in designing and testing a compact equipment which can recover thermal energy from low-temperature residual fluids at low costs of production and maintenance using termosyphons. Since the gravitational heat pipes is a passive technology and do not require any moving parts, the costs of recovering thermal energy are reduced. [PERSON] et al. [7] made a theoretical and practical research on three different approaches to implement a geothermal waste heat recovery system which is supplying the necessary heat to the pools of a fish farm. The results showed that when the most efficient method is used, the minimum costs of establishing a heat recovery system decrease by 60.65%. [PERSON] et al. [8] tried to improve the performance of the exhaust heat recovery from diesel engines by using a regenerative super-critical carbon dioxide Brayton cycle/organic Rankine cycle dual loop system. The results showed an improvement of the engine power output by 6.78%, which can be translated as a lower fuel consumption. A team of researchers made a numerical study on a waste heat recovery steam generator for hydrogen production. The research was aimed to improve the heat transfer mechanism of semi-coke and steam. The 7.5 kg/h proved to be the best steam flow for an efficient heat recovery rate [9]. Scientist have studied the possibility of introducing heat pipes in process of heat recovery for an improved efficiency of these type of systems. A heat pipe consists of a sealed enclosure, made from a heat conductive material and a working fluid. When heat is applied to the evaporator zone, the working fluid turns into vapor and transfers the heat to the condenser zone at a high rate. The vapors turn into liquid droplets and returns to the evaporator zone either with the help of gravity or through wick (Figure 1). Because the pressure inside the heat pipe is very low, the working fluid evaporates at a lower temperature than usual, making the heat pipe a super-conductor even at low temperatures. The most important benefit of heat pipes is that it is a passive way to recover thermal energy, which makes the process of waste heat recovery cheaper. Another advantage is  
Figure 1: The operating principle of heat pipes. their high conductance potential compared to common heat recovery technologies. Since each heat pipe functions individually, a heat pipe heat exchanger can operate properly even if one of them fails. Also, the fluids involved in the heat recovery process will not be mixed or contaminated in case of a heat pipe failure. The most common use of heat pipes is for solar applications. Studies have tried to improve the efficiency of heat pipes in solar collectors by testing different working fluids [10], by using thermal oil [11] or by adding fins to the condenser region of the heat pipes [12]. The optimum fill ratio is also an important factor on the efficiency heat pipes [13].