**6. Conclusion**

452 Heat Exchangers – Basics Design Applications

unit reflected that: a) Time to reach the target temperature of 44 °C increases: from 61 minutes it extends to 73 (19.7% improvement), being this a fundamental aspect of the application; b) The initial investment is reduced by 11%: from 3924 € to 3489 €; c) The PCM melting ratio is improved 23.2%; d) However, the volume occupied by the unit increments

Unit %Melt Investment [€] taditional, T=38ºC [min] taditional, T=44ºC [min] Δp [Pa] Proposed 92.64 3489 37 73 5 Optimized 100 3234 60 96 3

Dimensional analysis of these units show that the natural convection within the PCM is not going to be significant in any of the 2 units, being the heat transfer process by pure conduction for the second unit, and the ratio *λeff/λ* within the range of experimental validity for the other one (Dolado, 2011).Furthermore, since both Re and Bi numbers and NTU are

Keeping the temperature range, this type of heat exchanger can be applied in other different situations such as free-cooling, heat pumps, absorption solar cooling systems, greenhouses.

within the range of experimental validity, the units can be used for design purposes.

Table 13. Main results of the proposed and optimized units.

from 1.2 m3 to 3.8 m3.

Fig. 15. Optimization plot results.

**5.2.2 Model-prototype similarity** 

**5.3 Other applications** 

Methods to obtain enthalpy as well as the curves of thermal conductivity in the solid and liquid phases vs. temperature were proposed as a result of a critical analysis of the existing methods. A setup based on the T-history method was designed and built with significant improvements: 1) The possibility of measuring, for both organic and inorganic materials, cooling processes, therefore hysteresis and sub-cooling can also be studied; 2) The horizontal position decreases the error on enthalpy values since the liquid phase movements are minimized; 3) A Labview application allows the h-T curves to be directly obtained.

Results show that a heat exchanger using a PCM with lower thermal conductivity and lower total stored energy, but adequately designed, has higher cooling power and can be applied for free-cooling. Pressure drop is a key factor when designing any type of heat exchanger as it will determine the electrical energy consumption of the device. In the PCM-air heat exchangers with plates studied here, the pressure drop is ranged from 5 to 25 Pa. The analysis of the experimental data gathered accomplishes two aims: to develop empirical models of the TES unit and to come to a series of rules of thumb. Both are useful tools to design such kind of heat exchangers. For total energy storage strategy, the duration time of the cooling capacity of PCM heat exchanger depends on the cooling power demand. To validate the theoretical model developed, an uncertainties propagation analysis is proposed; here, the difference between the experimental and the simulation is less than 10% in terms of heat rate. The combined methodology of Design of Experiments applied to the numerical simulations seems to be a valid tool for design this kind of heat exchangers. When applied to the case study of temperature maintenance in a room, time to reach the maximum air temperature in the room was increased (19.7%), the initial investment was reduced by 11% and the PCM melting ratio was improved by 23.2%, as a drawback, the volume occupied by the unit was increased around 3 times.
