Design and performance of a xenon-based cryogenic heat pump demonstrator for future LXe observatories

TL;DR

Problem: XLZD requires high dynamic flow to purge $^{222}\mathrm{Rn}$; Approach: a xenon-based cryogenic heat pump demonstrator operating on a left-turning Clausius-Rankine cycle is built and tested; Findings: at $p_C=3.3$ bar and $p_C=4.3$ bar, the system delivers about 120 W cooling and 126 W heating with ~386–393 W input; COP values indicate substantial room for efficiency improvements, and scaling to a 25× larger system is in progress to meet XLZD's rigorous requirements; Significance: informs scaling decisions, control strategies, and integration prospects for radon-mremoval distillation in XLZD and related LXe detectors.

Abstract

This manuscript details the development and characterization of a small-scale cryogenic heat pump demonstrator, a technology designed to enable high-flow xenon distillation systems for the removal of $^{222}\mathrm{Rn}$ in future liquid xenon observatories like the XLZD experiment. The heat pump demonstrator operates on a left-turning Clausius-Rankine cycle, utilizing xenon as phase-changing working medium. Two demonstration tests conducted at a nominal pressure of $3.3\,\mathrm{bar}$ and $4.3\,\mathrm{bar}$ showed stable operation through out the test. In both measurements the demonstrator achieved its designed cooling and heating power of $(124\pm8)\,\mathrm{Watt}$ and $(126\pm8)\,\mathrm{Watt}$ respectively, while consuming $(386\pm1)\,\mathrm{Watt}$ electrical power.

Figures (5)

• Figure 1: Simplified schematic of the heat pump demonstrator representing the most important components: The condenser (1) with its cold head (CH) in green acts as a "cold reservoir" providing a fixed cooling power equivalent to the heating power $\dot{Q}_\mathrm{C}(\dot{m})$ of the heat pump and adjusted through heating elements (dark gray) with power $\dot{P}_\mathrm{C}(\dot{m})$. It is followed by an expansion valve (2) and the evaporator (3) in which additional heating elements serve as a "heat reservoir" providing an adjustable heating power $\dot{P}_\mathrm{E}$ equivalent to the cooling power $\dot{Q}_\mathrm{E}(\dot{m})$ of the heat pump. The gas then passes a gas-gas heat exchanger (HE) (4), and a compressor plus flow controller (5). Components which are filled with LXe (indicated in blue) are mounted inside an insulation cryostat depicted as gray shaded region. The remaining components are exposed to the ambient environment which serves as a constant heat bath. Red arrows indicate the external heat influx $\dot{Q}_j$ into the system, while black arrows show the heating and cooling power of the system. Light blue arrows represent the mass flow $\dot{m}_j$ for a given heat load. When merged with a cryogenic distillation column the cold head and heater of the condenser are replaced by the bottom part of the column filled with LXe (light blue dashed). The heater of the evaporator are replaced by the top part of the column. Top and bottom column parts are depicted as shaded components above and below the condenser and evaporator of the heat pump
• Figure 2: Left: CAD rendering of the heat pump demonstrator with a cutaway view into the condenser (top vessel) and evaporator (bottom vessel). The internal copper pins for improved heat transfer and the cylindrical capacitor level meters are visible. Pipes depicted in red and blue transport gaseous and liquid xenon, respectively. Pressure sensors are connected with condenser and evaporator through pipes colored in purple. The expansion valve body is highlighted in light blue. The outlet pipe of the exapnsion valve is connected to the bottom vessel at the cut-away CF40 connection. Right: Photograph of the cryogenic components installed inside the open insulation cryostat, showing the condenser, evaporator, and the connecting pipelines and cables. The image is rotated by about 45 with respect to the CAD model
• Figure 3: System parameters as function of time for the $p_\mathrm{C}(\qty{0}{\watt})=\qty{3.3}{\bar{}}$ performance measurement. If applicable either the first sensor reading or the average is subtracted from the shown parameters as indicated by the respective plot legend. Colored shaded regions represent periods over which the different parameters were averaged for a given heat load $P_E$ ranging between 90 and 130. The average for the 50 and 70 measurement was estimated as detailed in the text. The average values are depicted as horizontal lines, and the standard deviation of each period is indicated by the error bar at the data point in the center of an averaging period
• Figure 4: Pressure-enthalpy (left) and temperature-entropy (right) diagram of the Clausius-Rankine cycle for the 130 measurement. The two figure illustrate the different states of the heat pump's thermodynamic cycle listed in Table \ref{['tab:state_points']}. The gray dashed lines in the left figure indicate the assumed ideal compression described in the text. Red dashed-dotted lines show isothemal lines of different temperatures. Gray lines indicate the steam quality in 10 steps. The numbers corresponds to the process steps depicted in Fig. \ref{['fig:heat_pump_sketch']}. Two isobaric lines are depicted in the right hand side figure in blue
• Figure 5: The figure on the left and right hand side show the measured and calculated cooling and heating powers as function of xenon mass flow for the 3.3bar measurement, respectively. The lower panel shows the relative difference between measured and calculated heating and cooling power which approaches 8±2 for high mass flows