• No results found

Chapter 5: Conclusions and Recommendations of Future

and the lack of side insulation. Possible solutions for overcoming this may be to use extruded polystyrene insulation to match the collector profile. This would

‘slot’ into the underside eliminating air spaces. Any other improvements that reduce the heat loss from the collector, would contribute to increasing its thermal performance.

It was also decided that rather than optimise the collector, due to its simple construction, its viability for use in a domestic hot water heating system would be investigated. A comparison however, with a similar system concluded that the temperatures achieved by the system were relatively low. Moreover, a simulation model of the system using TRNSYS found that there was a good correlation between the predicted and actual, thus verifying a viable method for accurately predicting the system performance.

Since the current BIT collector design was considered as ‘poorly’ performing, a hypothetical case was also investigated using the TRNSYS model. In this case the heat losses in the collector were assumed to be comparable to that of a well performing integrated collector. The subsequent results of the simulation highlighted the importance of addressing the heat losses which significantly affect the collector’s performance, particularly for water heating. Additionally, simulated results for the improved BIT system showed that for a summer week the collector could meet a high percentage of the hot water requirements for a medium sized residence in Hamilton. Furthermore, it was observed that for a

winter week the BIT-SWH system performed well when compared to a BIPVT-SWH system.

Furthermore, a theoretical test of the control strategy for the BIT-SWH system showed that the strategy used was a viable configuration for the control of a water heating system. This was especially true for the control of the pump in the collector loop. The tests also showed a slight impracticality in the method used for controlling the load, it was suggested that if an alternative method was used a slight modification of the controller could be made due it’s the flexibility.

.

In summary this study has highlighted the potential for BIT collectors and BIT-SWH systems for meeting the heating requirements of New Zealand households, however careful consideration needs to be made into managing the heat losses of the collector and system. Additionally, the potential for controlling these systems was shown to be viable and provides the possibility for managing such systems to operate at increased efficiencies by using advanced control strategies.

5.2 Recommendations for Future Work

Aside from the remarks that have been made about improvements to the collector and system, there are a number of opportunities in the area of solar thermal, particularly building integrated and domestic water heating, for which further work can be applied.

New Zealand’s closest neighbour, Australia, is a world leader in the solar thermal industry. Therefore, renewable derived energy systems are essentially a large part of the development within the country. It would be of interest to compare how such a widespread adoption of solar technologies has occurred and how this can be achieved in New Zealand. A study investigating the possible barriers faced by New Zealand can provide interesting insight.

It was observed that for this integrated collector, the high level of integration that was achieved, boarders into the building and construction industry. If perhaps the technology is to be adopted, from a technical point of view an investigation into how such systems would fit into the current building regulations or if modifications are required, may prove useful. That being said, this would also relate directly to the heat and mass transfers that occur within the ‘built’

environment. In this context, if the BIT is used on a standard metal long run roof, an aluminium heat absorbing material is in ‘direct’ contact with standard roofing coloured steel. The implications of having heat transfer by having metals with

Additionally, the thermal expansion of roofing structures is a phenomenon observed in the roofing industry, a study into how this would be accounted for with integrated collectors would provide useful insight.

Finally, it was noted that there is currently no set configuration for integrated water heating systems. An optimisation study of systems relating to integrated collectors will significantly aid in the widespread adoption of architecturally uniform collectors and systems. It is also recommended that the area of system control be seriously addressed as it provides further opportunity for acceptance of solar water heating systems. A possible approach to this statement would be to use advanced control strategies.

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Appendix A: Tested BIT and BIT-SWH parameters

Table 2: Experimental BIT physical characteristics.

Parameter Symbol Value Unit

Number of covers N 1 or 0

Number of channels per panel n 2

Channel diameter D 5 mm

Channel Spacing W 72 mm

Hose diameter dh 20 mm

Collector Length Lc 2 m

Collector Width wc 1.13 m

Collector Area (Gross) AG 2.26 m2

Absorber thickness t 3.75 mm

Absorber Conductivity (Dals Steel

Metal Pty Ltd, 2011) kA 209 W/mK

Insulation Thickness Lin 50 mm

Insulation Conductivity (Bondor, 2013) ki 0.037 W/mK

Mounting Angle β 37 °C

Table 3: Experimental BIT-SWH physical characteristics

Component and

parameter Symbol Value Unit

Collector

Number of covers N 1 or 0

Number of channels

per panel n 2

Channel diameter D 5 mm

Channel Spacing W 72 mm

Hose diameter dh 20 mm

Collector Length Lc 6 m

Collector Width wc 1.13 m

Collector Area

(Gross) AG 6.78 m2

Absorber thickness t 3.75 mm

Absorber

Conductivity (Dals Steel Metal Pty Ltd, 2011)

ka 209 W/mK

Insulation Thickness Lin 50 mm

Insulation Conductivity (Bondor, 2013)

kin 0.037 W/mK

Mounting Angle β 5 degrees

Drain back tank

Volume Vdb 45 L

Maximum working

pressure pmax 76 kPa

Storage tank

Tank volume Vst 172 L

Standing heat loss tloss 1.6 kWh/day

Pump

Maximum power Pmax

90 W

Appendix B: Collector and System Testing Schematics

Figure 52: Schematic of BIT collector testing rig.

Figure 53: Schematic of BIT-SWH system testing rig.

Appendix C: Collector Testing Experimental Data

Table 4: Unglazed BIT collector data

Ambie nt Temp

(°C)

Irradiance (W/m2)

Inlet Temp

(°C)

Outlet Temp

(°C)

Mass flow rate

(kg/s)

Instantane ous heat

gain (Q)

Instantan eous efficienc

y

23.5 936.6 27.1 37.5 0.02 873.4 0.39

23.4 938.1 27.1 37.7 0.02 894.0 0.39

23.3 939.1 27.1 37.8 0.02 902.0 0.40

23.3 940.7 27.1 37.7 0.02 892.7 0.39

23.3 942.0 27.1 37.6 0.02 882.2 0.39

23.3 943.0 27.1 37.3 0.02 854.6 0.37

23.3 943.5 27.2 37.6 0.02 877.2 0.38

23.3 943.7 27.2 37.3 0.02 852.5 0.37

23.3 945.3 27.2 37.2 0.02 845.0 0.37

23.2 946.3 27.2 37.6 0.02 876.1 0.38

23.3 946.6 27.2 37.7 0.02 884.1 0.39

23.3 947.5 27.2 37.6 0.02 877.9 0.38

23.5 950.2 27.2 37.8 0.02 890.6 0.39

23.6 948.9 27.3 38.1 0.02 907.4 0.40

23.7 949.6 27.3 38.4 0.02 929.7 0.40

23.8 949.1 27.4 38.2 0.02 913.3 0.40

23.9 951.2 27.4 38.5 0.02 936.8 0.41

23.8 951.1 27.3 38.5 0.02 940.2 0.41

23.8 950.9 27.3 38.6 0.02 944.4 0.41

23.8 953.7 27.4 38.7 0.02 953.5 0.41

23.8 953.7 27.4 38.5 0.02 937.4 0.41

23.9 952.5 27.4 38.7 0.02 947.9 0.41

24.0 957.3 27.6 38.5 0.02 916.3 0.40

24.0 955.2 27.5 38.3 0.02 907.9 0.39

23.9 955.5 27.5 38.5 0.02 920.1 0.40

24.0 954.8 27.6 38.6 0.02 924.8 0.40

24.2 957.6 27.7 38.5 0.02 913.2 0.39

24.2 957.5 27.7 38.2 0.02 884.1 0.38

24.2 959.1 27.7 38.4 0.02 892.2 0.38

26.3 1016.7 49.2 50.6 0.02 121.0 0.05

26.3 1018.1 49.2 50.8 0.02 134.7 0.05

26.2 1021.3 49.2 50.7 0.02 130.7 0.05

26.3 1022.4 49.1 50.8 0.02 140.0 0.06

26.4 1022.9 49.1 50.8 0.02 142.2 0.06

26.4 1023.7 49.0 50.9 0.02 159.3 0.06

26.3 1022.6 48.9 51.0 0.02 169.2 0.07

26.3 1023.4 48.8 51.1 0.02 188.1 0.08

26.2 1022.4 48.7 50.9 0.02 181.8 0.07

26.2 1023.7 48.7 50.9 0.02 187.7 0.08

26.3 1022.9 48.7 51.0 0.02 198.2 0.08

26.3 1024.0 48.7 50.8 0.02 173.5 0.07

26.3 1022.4 48.8 50.8 0.02 168.0 0.07

26.1 1021.4 48.8 50.7 0.02 164.8 0.07

26.0 1021.9 48.9 50.6 0.02 144.7 0.06

26.0 1019.8 48.9 50.5 0.02 131.9 0.05

26.1 1019.4 49.1 50.6 0.02 124.6 0.05

26.0 1020.1 49.2 50.5 0.02 110.3 0.04

26.3 1018.1 49.3 50.5 0.02 107.3 0.04

26.6 1019.1 49.0 50.3 0.02 109.3 0.04

26.6 1020.1 49.1 50.5 0.02 114.7 0.05

26.5 1019.4 49.1 50.4 0.02 105.7 0.04

26.4 1021.4 49.2 50.5 0.02 114.1 0.05

26.5 1022.2 49.2 50.6 0.02 117.9 0.05

26.6 1021.1 49.3 50.8 0.02 123.7 0.05

26.6 990.3 57.3 57.9 0.02 44.0 0.02

26.7 993.2 57.2 58.0 0.02 63.8 0.03

26.7 988.8 57.0 58.1 0.02 89.1 0.04

26.8 986.8 56.9 58.2 0.02 104.2 0.04

26.8 985.8 56.9 58.2 0.02 109.2 0.05

26.8 983.5 56.9 58.1 0.02 100.3 0.04

26.9 982.9 57.1 58.0 0.02 80.0 0.03

26.8 982.5 57.3 58.1 0.02 69.2 0.03

26.6 982.9 57.5 58.2 0.02 59.9 0.03

26.5 983.4 57.8 58.2 0.02 39.3 0.02

26.2 983.9 57.0 58.2 0.02 96.3 0.04

26.3 979.1 57.3 58.2 0.02 79.8 0.03

26.5 979.1 57.5 58.1 0.02 52.1 0.02

Table 5: Glazed BIT collector data

Ambien t Temp

(°C)

Irradianc e (W/m2)

Inlet Temp

(°C)

Outlet Temp

(°C)

Mass flow rate (kg/s)

Instantaneou s heat gain

(Q)

Instantan eous efficienc

y

27.6 996.5 27.8 32.8 0.097 2001.5 0.83

27.6 998.8 27.8 32.6 0.097 1952.7 0.81

27.6 998.3 27.8 32.6 0.097 1925.4 0.80

27.6 998.0 27.8 32.5 0.097 1879.6 0.78

27.6 1002.1 27.8 32.4 0.097 1873.1 0.77

27.7 1001.4 27.8 32.4 0.097 1881.7 0.78

27.7 1001.6 27.8 32.4 0.097 1869.6 0.77

27.8 1003.4 27.8 32.4 0.097 1854.6 0.76

27.8 1001.6 27.9 32.4 0.097 1842.9 0.76

27.9 1000.1 27.9 32.4 0.097 1832.3 0.76

27.8 1002.6 27.8 32.4 0.097 1848.0 0.76

27.6 1002.2 27.9 32.4 0.097 1830.9 0.75

27.6 1001.1 27.9 32.4 0.097 1834.8 0.76

27.5 1001.7 27.9 32.4 0.097 1828.3 0.75

27.3 1002.7 27.9 32.4 0.097 1825.1 0.75

27.3 1003.4 28.0 32.5 0.097 1829.7 0.75

27.3 1000.1 28.0 32.5 0.097 1827.3 0.76

27.2 1009.4 28.0 32.5 0.097 1810.9 0.74

27.0 1009.0 28.1 32.5 0.097 1812.8 0.74

26.8 1008.8 28.1 32.5 0.097 1814.1 0.74

26.7 1012.6 28.1 32.6 0.097 1822.9 0.74

26.7 1016.2 28.0 32.5 0.097 1821.6 0.74

26.7 1013.9 28.1 32.5 0.097 1812.7 0.74

26.7 1014.4 28.1 32.5 0.097 1795.3 0.73

26.9 1017.8 28.2 32.6 0.097 1793.8 0.73

29.0 1000.9 44.6 46.8 0.097 903.3 0.37

29.3 1005.8 44.6 46.7 0.097 869.3 0.36

29.4 1005.0 44.5 46.6 0.097 843.3 0.35

29.5 1004.0 44.5 46.6 0.097 846.0 0.35

29.5 1004.0 44.5 46.5 0.097 819.7 0.34

29.5 1001.9 44.4 46.4 0.097 808.6 0.33

29.5 1003.4 44.4 46.3 0.097 774.2 0.32

29.5 1001.6 44.4 46.3 0.097 758.0 0.31

29.6 1003.2 44.5 46.3 0.097 740.3 0.30

29.7 1006.0 44.4 46.3 0.097 735.6 0.30

29.6 997.3 44.5 46.4 0.097 731.3 0.30

29.5 1002.4 44.6 46.5 0.097 745.5 0.31

29.6 1005.7 44.6 46.3 0.097 707.9 0.29

29.6 1002.7 44.5 46.1 0.097 651.4 0.27

29.4 1033.2 56.1 57.2 0.097 462.0 0.18

29.2 1033.2 56.3 57.3 0.097 391.1 0.16

28.9 1041.4 56.4 57.2 0.097 353.2 0.14

28.8 1035.5 56.4 57.2 0.097 327.0 0.13

28.8 1036.8 56.4 57.3 0.097 339.7 0.14

28.6 1033.7 56.6 57.3 0.097 313.2 0.13

28.7 1033.4 56.4 57.3 0.097 361.4 0.14

28.8 1033.1 56.4 57.2 0.097 326.8 0.13

28.9 1030.6 56.4 57.2 0.097 317.9 0.13

29.0 1036.2 56.3 57.2 0.097 337.7 0.13

29.1 1036.3 56.3 57.1 0.097 324.7 0.13

29.1 1037.5 56.1 57.0 0.097 359.6 0.14

29.0 1038.6 56.0 56.9 0.097 378.5 0.15

28.8 1040.8 55.8 56.8 0.097 413.1 0.16

28.7 1037.2 55.6 56.8 0.097 454.9 0.18

28.6 1036.2 55.6 56.8 0.097 475.9 0.19

28.6 1038.8 55.5 56.7 0.097 510.9 0.20

28.6 1043.1 55.5 56.9 0.097 586.4 0.23

28.7 1037.2 55.7 57.0 0.097 535.9 0.21

28.7 1036.2 55.6 56.9 0.097 564.0 0.22

28.8 1033.6 55.5 56.9 0.097 576.8 0.23

28.9 1034.9 55.6 56.9 0.097 529.7 0.21

29.1 1032.1 55.5 56.9 0.097 581.4 0.23

29.0 1034.9 55.5 57.0 0.097 596.0 0.24

Appendix D: Algorithm for Pump and Solenoid Control

The pump controller was controlled used a temperature differential algorithm. The program was developed using NI’s development software, labVIEW. The program structure was based on a solid state machine.

The program was split into three states for the pump control that is, the start state (in this instance the default state), pump on state and the pump off. Each state is illustrated below and the logic for activation explained.

Start state, shown in Figure 54 is when the program is ‘actively’ waiting until its algorithm can be executed. For the activation to occur, three requirements are to be met is firstly, when the collector outlet, Tco, is greater than the temperature in the cold region of the tank, Ttank, indicating that there is heat to be ‘gained’ from the collector. Secondly the temperature difference between the collector outlet and collector inlet, Tci is greater than 8 °C. The last requirement is that the temperature in the hottest region of the tank, Tt1, needs to be less than 90 °C to avoid overheating.

Figure 54: Start state for pump controller

Once these conditions are met the machine moves to the ‘pump on’ state, shown in Figure 55. When the program enters this state a TRUE signal is sent to the relay to activate the pump. The program then ‘actively’ checks if the requirements of the algorithm are met. Three requirements are needed, firstly the Tco > Ttank so that there is always heat gain. Secondly, the inbuilt safety to account for overheating, Tt1 < 90 °C. The last requirement is when Tco – Tci < 4 °C, when this occurs the program state shifts to the next state.

The final state, ‘Pump off’’ state sends a FALSE signal to the relay to switch the pump off, shown in Figure 56. The state machine then returns to the ‘Start’ state and the program waits to activate again when needed.

Figure 56: Pump off state of the pump controller

For the solenoid control, a solid state machine program similar to the pump controller was developed; however as can be seen from Figure 57 a more complex algorithm is needed to simulate the load. Unlike the pump controller, which is activated solely on parameters from the system the ‘dump’ controller is activated by the local time and load requirement as set out in the standardised load profile.

In summary when a specific time period, requiring a specific load, is activated the program moves to the ‘True’ state where is sends a TRUE signal to the solenoid valve.

Figure 57: Block diagram of solenoid controller

The program then actively calculates the heat extracted, Qtank, from the tank using Equation 27.

Qtank = m Cp (Texit – Tmain) (27)

Where, m is the flow-out of the tank to the load, Cp is the heat capacity of water and Texit and Tmain are the outlet and inlet tank temperatures, respectively. The reason for this measure was because the hourly load profile is based on energy required per hour (MJ/hr) as opposed to a certain amount of litres.

When the solenoid is activated by time and required energy, the heat extracted from the tank and energy required are ‘actively’ compared. Once the required energy is drawn from the tank the controller sends a FALSE signal closing the solenoid and returns to its base state where it waits for the local time to activate is again.