Final Report Summary - HP-LP-SOLAR-FACADE (A Novel Heat Pump Assisted Solar Façade Loop Heat Pipe Water Heating System)
1. Conceptual design of the proposed heat-pump assisted solar façade loop heat pipe (LHP) water heating system.
2. Development of the computer models to optimise the system configuration and predict its operational performance.
3. Construction and testing of a prototype system in laboratory.
4. Economic, environmental and regional acceptance analyses.
Detailed technical information including a diagrammatic project plan is given in the attached document. Over the two-year project duration, all the above tasks have been successfully completed and these are briefed as follows:
Task 1 Conceptual design of the proposed heat-pump assisted solar façade LHP water heating system
The conceptual design of the proposed heat-pump assisted solar façade LHP water heating system was completed on basis of a 5-family member flat, involving preliminary selection, sizing and positioning of the major system components. This system contains the outdoor and indoor parts that are linked together via the vapour/liquid transporting lines. The outdoor part, structured as the modular building façade (wall or balcony) incorporating the glass cover and wicked heat absorbing pipes and feeders (part of the LHP), is designed to absorb solar energy and vaporising the heat transfer fluid within the heat absorbing pipes. The indoor part, comprised of the secondary water tank, primary water tank, heat-pump and water piping, can convey the absorbed heat into the service water through the condensation of the evaporated heat transfer fluid.
Estimation on the quantity and sizes of the system components has been undertaken. The collector area is around 2 m2, within which around 15 mesh-screen wicked pipes with the diameter ranging from 0.013 to 0.019 m are incorporated. A number of single flat-plate glazings are compared in terms of their geometrical, optical and thermal properties and the best suited one is selected. Furthermore, the design and operational parameters of the heat-pump and water tanks are pre-selected, the input power of the heat-pump is estimated to be 1kW and the volumes of the primary and secondary water tanks are set as 0.2 and 0.04 m3 respectively. Taking these estimated data as the starting point, simulation of the system performance is undertaken as part of task 2 works, thus leading to determination of the system’s design and operational parameters.
Task 2. Development of the computer models to optimise the system configuration and predict its operational performance
Task 2 involved the following works: (i) CFD simulation of the fluid flow in the heat absorbing pipes (evaporation sections of the loop heat pipe); (ii) selection of the working fluids using a computerised analytical model developed by us; (iii) investigation of the impact of the operational parameters and determination of the favourite system configuration and operational conditions, by using a steady-state computerised numerical model developed by us; (iv) investigation of the day-time operational performance of the system, by using a computerised dynamic numerical model developed by us.
The dedicated CFD simulation provided the profiles of the fluid flow within the wicks and vapour areas in the heat absorbing pipes. This indicated that the velocities at the inlet and outlet sections of a heat absorbing pipe appear to be higher than the velocities at any other sections. In the same cross section of the heat absorbing pipe, axial (Z-) directional velocities in both wicks and vapour area are found to be significantly higher than those at other directions, indicating that the larger liquid/vapour flow-rates are in existence along this direction.
Selection of the working fluids should consider both thermal performance and operational feasibility. Water, although able to achieve the highest heat output and thermal efficiency, has found a certain problem in practical application, i.e. difficulty in maintaining a lower than atmospheric pressure (vacuum) to enable evaporation to take place at 30oC operational condition. Use of a refrigerant (e.g. R600a, R134a and R22) could prevent this problem as its operational pressure at 30oC is above the atmospheric pressure. Comparison among three refrigerants indicated that R600a would be the most feasible choice for the proposed heat-pump assisted solar façade LHP water heating system.
Impacts of the operational parameters on the system performance were investigated. It is concluded that higher solar radiation, higher air temperature, lower air velocity, smaller cover number, lower heat-pump evaporation temperature and larger number of heat absorbing pipes gave rise to the system’s solar thermal efficiency. To achieve a better operational performance for the heat-pump assisted solar façade LHP water heating system, the façade module should comprise a single glazing cover and at least two heat absorbing pipes in a 0.6m2 area. During the operation of the system, the evaporation temperature of the heat-pump is suggested to be 10oC. A 2.4 m2 of such a solar façade module can heat up 120 litres of water from 15oC to 55oC under the London summer weather condition.
A computerised dynamic numerical model has been developed to simulate the day-time operational performance of the proposed system under the typical London summer weather condition. It is suggested that: (1) system COP would vary in a range 3.27 to 8.67; (2) system can raise temperature of water from initial 15oC to final 40.5oC at 5pm; (3) to obtain an even higher water temperature, the solar collector area should be increased to 3.0 m2, instead of the initially suggested 2 m2.
Task 3. Construction and testing of a prototype solar façade system in laboratory
In this task, an experimental prototype was constructed and tested under the laboratory condition with the aim of examining the time-dependant performance of the system. The justified testing conditions are: solar radiation of 626 W/m2, controlled ambient temperature of 20oC, heat-pump switch-on temperature of 35±0.5oC and switch-off temperature of 25±0.5oC which represent a clear sky summer daytime weather condition in London. Meanwhile, the established computer model was also applied to simulate the performance of the system under the equivalent condition to the test. Comparisons between the modelling and experimental results were undertaken, suggesting that the model could achieve acceptable accuracy in predicting the system’s operational performance, with the error scale in the range 1% to 11.7%. Under the above justified condition, the system could provide the hot water of around 55oC that can meet the general domestic requirement. This system could obtain a solar thermal efficiency of around 71% and had an average COP of around 4.95.
Task 4. Economic, environmental and regional acceptance analyses
In this task, the feasibility and economic & environmental benefits of the new system for application in Europe buildings were investigated. The feasibility study involved the analyses of the EU weather data relating to the standard design year and assessment of the water generation capacity of the system in different European regions. The economic and environmental benefits assessment included (1) analyses of the capital and operational cost of the heat-pump assisted solar façade LHP water heating system; (2) calculation of increase in the capital cost and saving in operational cost of the system relative to the conventional water heating systems; and (3) estimation of the payback period and life cycle cost saving of the system relative to the conventional ones. Furthermore, the carbon emission reduction potential of the system for the use as a replacement of the conventional water heating systems across the European regions was analysed. It is concluded that the heat-pump assisted solar façade LHP water heating system is suitable for the southern Europe region, but less applicable for the northern Europe region. The temperature of the primary tank water in such a system could achieve above 50oC during most period of the year in Madrid, indicating that Madrid is the most suitable place for application of the technology. If this system is installed in London, the duration that can obtain the above required water temperature is July only. If this system is installed in the cold northern European region, e.g. Stockholm, the highest water temperature that the system can achieve is 47oC, suggesting that such a system cannot be independently and economically operated in this region and an additional heating measure is always in need across the full year. Compared to the conventional water heaters, the new system could achieve the reasonably fast payback time, i.e. 5.91 6.36 and 6.91 years in Madrid, London and Stockholm, which are the acceptable figures compared to the system’s 15 years life span. For a 2 m2 façade based system, associated fossil fuel cost savings relative to a conventional heating system were around €3,210, €3,060, and €2,640 respectively in Madrid, London and Stockholm; while the associated carbon emission reductions were 19.7 tonnes in London, 18.3 tonnes in Stockholm, 19.6 tonnes in Madrid over the 15 years of life cycle period.