| Abstract | Heat transfer fluids (HTFs) are an essential heat transport medium in many wet heating and cooling systems. They carry Heat from the generation source to the place where it is being used. Therefore, HTFs should have the capability of carrying Heat efficiently. The ability of HTFs to carry out the maximum amount of Heat with minimum loss is dependent on the characteristics and properties of the fluids; better heat transfer will result in better heating system performance. The majority of conventional heat transfer systems, within the built environment, currently use water as the heat transfer medium between the source and the point of use. This is mainly due to the availability, low price and acceptable thermal properties of water. However, water has its limitations in terms of heat transfer rates in intense energy systems due to aeration, oxidation and fouling. In addition, it has a high freezing point (0°C) which affect its use in a cold climate. Therefore, using alternative heat transfer fluids is considered as a choice to overcome the limitations that are associated with water as a HTF.
The conducted literature review has demonstrated a lack of knowledge in the use of alternative heat transfer fluids, especially nanofluids in hydronic radiator heating systems
(HRHS) and the effect of these fluids on the overall system energy performance. Therefore, this research was designed to experimentally investigate and compare the energy performance of a radiator heating system with alternative heat transfer fluids compared to water. For this research project, a commercially available nano-based heat transfer fluid (50%HX/W) was researched and compared using energy performance tests to potable water, 30% ethylene glycol/water (30%EG/W) and 50% ethylene glycol/water (50%EG/W) mixtures. To achieve the target of this research a bespoke test facility (simulating a residential heating system) was designed, constructed and fully instrumented to investigate the performance of the hydronic heating system with the different options of heat transfer fluids under similar and repeatable controlled conditions. Test methods were developed for both steady state and thermostat tests and two independent scenarios; Drop-in scenario and optimised scenario. The drop-in-scenario replicates a situation where the alternative fluids are charged into the system without
changing the system settings (flow rate and temperatures settings). The optimised scenario replicates a situation where the alternative fluids are charged into the system and adjustment are made to the settings, including the mass flow rate and temperature difference (∆T) across the radiator. These scenarios were designed to test the radiator heating system performance with the alternative heat transfer fluids (AHTFs) under investigation to obtain the required results. System energy data, air and system temperatures data, as well as heat output from the radiator, were considered for the
evaluation and comparing of the system performance with all examined fluids.
In order to evaluate the heat output from the radiator heating system working with the examined AHTFs, an energy balance approach was developed during this project. This approach allows evaluation of the heat output from the radiator of the considered heating system to the air-side during steady-state and transient conditions without considering the dynamic changes in the thermal properties of the working fluid at different temperatures.
The results obtained show that the properties of the examined fluids have an impact on system operational behaviour and performance. For the drop-in scenario, test results revealed that the system flow rate was related to the density and viscosity of the working fluids. The flow rate in the system was lower (compared to water) when 30% and 50% EG/W and 50% HX/W were used in the system as follow, 9%, 32% and 46% respectively. The internal booth temperature (IBT) and energy consumption obtained from 30% EG/W test were very close to the results obtained from the water test. While lower IBT (by 1.4K – 2.5K) was obtained when 50% EG/W and 50% HX/W used as a working fluid in the radiator compared to the base case (water test) value. Comparison data of energy consumption showed that less energy was consumed when 50% EG/W and 50% HX/W used in the heating system, and this reduction was 1.7 kWh and 2.6 kWh respectively, during the duration of the test. The test results of the optimised scenario revealed that the adoption of the radiator design mass flow rate with 50% EG/W and 50% HX/W tests resulted in a lower volumetric flow rate. This resulted in a bigger ∆T across the radiator (by 18%), better temperature uniformity on the radiator surface and lower return temperature (by 2K) compared to the water test. Considering the 50% EG/W and 50% HX/W as a working fluid in the radiator heating system with the option of design ∆T across the radiator (10K) resulted in a higher mass flow rate in the system (by 17%) compared to water test.
For all optimised tests, it was noted that using 30% EG/W as working fluids in the heating system resulted in similar behaviour to the base case.
This research has contributed to knowledge through the followings:
• A bespoke test facility and methodology for conducting repeatable tests to evaluate the performance of a HRHS when using different heat transfer fluids under the same environmental conditions.
• An innovative approach (based on the energy balance principle) to evaluating the heat output from a radiator when operating with AHTFs. This approach allows assessing the heat output during steady state and transient conditions, as well as during the period when the heating system is 'off'. Also, it helps to overcome the limitations of the BS EN 442-2 model, which is only applicable to radiators that operate with water or steam.
• A better understanding of the performance of radiator heating systems when using alternative fluids in terms of flow, heat transfer and energy. |
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