For many applications, the possibility of controlling heat flow by “thermal switching” can be very beneficial. In previous work, we presented a novel approach for thermal switching using a water-loaded adsorbent as part of the evaporator of a heat pipe. The basic idea is that the adsorbent releases water upon exceeding a certain evaporator temperature and thus “activates” the heat pipe by providing the working fluid for thermal transport. In this work, we present an improved version of the heat switch. We found that an ordinary copper heat pipe (i.e., thermosyphon) with outer diameters common in heat pipe applications (10 mm) can be used as a base of the heat switch. The reversibility of the switching effect was proven. The location of heat input relative to the adsorbent position was optimized, leading to improved switching ratios of up to 36 as calculated from thermal resistance change between evaporator and condenser. Furthermore, the role of the insulation on thermal resistance and switching ratio was examined and found to be significant.

An experimental investigation was conducted on a small-bore, high-speed diesel engine to study the effect of different operating parameters on combustion and engine-out emissions in the conventional and low temperature regimes. For the conventional diesel combustion, the spray behavior is analyzed and a differentiation is made between the conditions in the small-bore and the larger bore quiescent chamber engines. The effects of the injection pressure, exhaust gas recirculation (EGR), injection timing and swirl ratio (SR) on combustion and engine-out emission are investigated. The trade-off between NOx and smoke, measured in Bosch smoke unit, (BSU), is investigated with a special attention to the low temperature combustion regime, (LTC). The results showed that the LTC regime could be reached at fairly high EGR rates under all the injection pressures investigated in this work. The margin for the variation in EGR was limited just before the misfiring EGR. A small increase in EGR to the firing limit resulted in a drop in both the NOx and BSU, with a noticeable penalty in fuel economy, and an increase in HC and CO emissions. The increase in swirl ratio was effective in reducing both NOx and BSU in the LTC regime, but with some penalty in fuel economy.

Recently, thermal regulators based on adsorption in a heat pipe have been proposed. The advantage of these so-called “switchpipes” over similar approaches is their low on state thermal resistance. In this paper, we propose a methodology to calculate and adjust the activation temperature of such switchpipes. For this purpose, we use a mass balance-based model that considers both the heat transfer properties of the heat pipe itself, which depend on the amount of working fluid, and the adsorption equilibrium of the adsorbent used. This model can be used not only to describe the activation behavior of a given heat pipe but also to optimize the configuration of a heat pipe for specific operating conditions and to select appropriate adsorbents. In this paper, we also propose definitions for basic indicators of the activation properties of the heat pipe, such as the activation temperature and the activation temperature span. Finally, a simplified calculation method is presented that allows the selection of the correct adsorbent among all adsorbents with Type IV and Type V adsorption isotherms.

Controlling heat transfer through components with adjustable thermal resistance can be of great benefit in a wide range of applications such as the thermal management of spacecraft or electric vehicles. A novel concept for both thermal switching and thermal regulation is the use of a water-loaded adsorbent within a reservoir that a regular heat pipe is expanded with. By reversibly desorbing or adsorbing water, states of low and high thermal resistance can be achieved. This concept has been studied so far only in thermosiphons that rely on gravity support. To expand potential application fields, we successfully investigated the utilization of heat pipes with a capillary structure, achieving against-gravity operation. Adsorption-based heat pipe demonstrators were experimentally examined regarding their characteristic properties. Thermal resistances during the on and off state of 0.25 KW⁻¹ and 6.5 KW⁻¹, respectively, were measured, yielding switching ratios of up to 26. Furthermore, the role of the adsorbent reservoir heat exchanger was examined and found to have a significant potential to yield an improvement with regards to dynamic performance. With an improved demonstrator design, the dynamic performance was enhanced as the hysteresis behavior was reduced and a minimum switching time of 5 min was recorded.

Hysteresis and the associated production of dissipative heat during first order phase transitions are often major contributors to thermodynamic losses in caloric heat pumps. The figure of merit (FOM), defined as the ratio of adiabatic temperature change and the thermal hysteresis of the caloric material, quantifies these losses, and can also be used to calculate the maximum potential efficiency of a caloric material in a thermodynamic cycle. This paper presents a novel and simple method to determine the heat loss and thus the FOM can be determined from self-heating of the caloric material during repeated field cycling. As this method mainly requires temperature readings and the ability to cycle the caloric material in a field, most test setups that directly measure the adiabatic temperature change should already be able to perform dissipative heat measurements with this technique. With the presented method, we were able to determine the efficiency of a commercial LaFeSiMnH-sample with a high degree of accuracy. A maximum FOM of 37 ± 1 was determined for the selected LaFeSiMnH-sample. In an ideal cascaded magneto caloric system, this corresponds to a system efficiency of 90%, with an ideal heat regeneration this could theoretically even be increased to 97%.

Due to climate change, the need for efficient heat pumps is crucial. Magnetocaloric heat pumps are one of these potentially more efficient candidates. Magnetocaloric heat pumps use solid state refrigerants that exhibit a temperature change when a magnetic field change is applied to them. However, current prototype systems are still too expensive for commercial use, due to the need for large permanent magnets. By improving power density, the size of the magnets and therefore the cost can be reduced. To this date the highest power density was achieved by Maier et al (2020 Commun. Phys. 3 1–6) using the novel magnetocaloric heat pipe concept. In this work we improve on the design by building magnetocaloric heat pipe consisting of seven segments connected by check valves. Both ethanol and methanol were tested as the heat transfer fluids. Using 154 g of Gadolinium temperature differences of up to 11.5 K and cooling powers of 128 W were achieved. This is a significant step forward in both maximum cooling power as well as temperature difference compared to previous devices of this kind. An extraordinarily large working frequency of 14 Hz was achieved for this device, leading to a very high power density with a specific refrigerant capacity s R C P of 15 W kg⁻¹. In this work we also propose a simple lumped parameter model, which can predict the system performance and allows for further optimization of caloric heat pipes.