Experimental Apparatus
A schematic diagram is shown in Figure 2, depicting the main components of the ISAHP test rig. Table 1 lists the instrumentation and monitoring equipment used in the experiment, whose part numbers correspond to the schematic diagram.
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Table 1 List of instrumentation used
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The main components of the apparatus include: a nominal 1/3 HP single speed compressor, a thermostatic expansion valve, two flat plate counter-flow heat exchangers acting as the evaporator and condenser of the heat pump, a standard residential hot water tank (270 L), a variable speed pump and an auxiliary heater for simulating the solar collector heat input. R-134a was used as the working fluid for the heat pump cycle, and a 50 / 50% glycol/water solution by volume was used in the collector loop.
The collector loop operates in a similar fashion to that of a typical solar domestic water heating system. First, the glycol solution is pumped through the collector, or in this case the auxiliary heater, in a closed loop. The glycol solution absorbs energy through the “collector”, and a heat exchanger is used to extract the heat from the glycol, which acts as the evaporator of the heat pump loop. The R-134a superheated gas exits the evaporator and passes through the compressor, increasing in both pressure and temperature. The refrigerant then releases its heat and condenses through the natural convection heat exchanger, which delivers hot water to the storage tank. The R-134a liquid then passes through the thermostatic expansion valve, reducing its pressure before re-entering the evaporator.
2. Experimental Procedure
To compare the dynamic operation of the ISAHP system with the steady-state computer model [4], a 6 hour test with varying glycol temperature was performed. Due to stratification, the temperature at the bottom of the tank remained constant throughout the test providing a constant input temperature for the condenser on the water side. A description of the experiments performed is given below.
2.1. Test Sequence
For the simulated solar day test, the heat out of the auxiliary heater was varied in a sinusoidal profile similar to that occurring on a clear sunny day. The following procedure was followed for the test:
• Prior to testing the storage tank was filled with water at mains temperature to ensure that the entire tank was at constant temperature.
• During this time the collector loop fluid was brought to the desired initial temperature for the test, and the loop flow rate was set to 77 kg/h (0.021 kg/s). This flow rate was used in the simulations previously undertaken, and is based on recommendations by Fanney and Klein [14].
• The data acquisition (DA) system was initialized with the compressor running, and the program delivering the power profile to the heaters was commenced. Data was recorded every 5 seconds for the duration of the experiment.
• COP and natural convection flow rate were then calculated based on the collected data.
