Chapter 7 Conclusions and Recommendations for Future Work
7.2 Recommendations for Future Work
The current work is the first full experimental evaluation and implementation of a solar driven liquid desiccant air conditioning system at Queen’s University. Future development of the project involves three main areas of study: investigation of desiccant storage and improved control schemes, operation of the system with more realistic cooling loads, and the investigation of annual solar combi-system performance. In addition, the performance recommendations outlined in Section 6.4 should be addressed.
Desiccant storage has the potential to significantly increase the performance of the system. Implementation of desiccant storage should first be investigated using TRNSYS
118
simulations to determine the optimal control scheme and storage volume. This will likely require the development of a new TRNSYS component which can model a desiccant storage tank stratified by concentration. Storage can then be applied to the experimental demonstration to quantify the performance benefits. Additionally, advanced control schemes should be investigated through simulations including the implementation of efficient variable speed pump control to reduce parasitic energy consumption.
The next step in the development of this full-scale demonstration is to provide air- conditioning to a real building. Monitoring the system when servicing a real cooling load will highlight operational considerations not investigated in the current study. This advancement also has the potential to improve performance, since dry building exhaust air can be used as the regenerator scavenging air stream, improving regenerator effectiveness.
A solar combi-system, which provides space heating, hot water, and air-conditioning, allows solar collectors to be used year-round, improving performance of the system. The evacuated tube solar array was monitored over the winter of 2011/2012 and the energy collected (18,800 kWh) represents the upper estimate of the available energy for space heating. It is recommended that simulations be used to investigate potential configurations for a solar combi- system providing heating in the winter and using the LDAC for summer operation. The performance of the system can then be evaluated annually, and a comprehensive economic analysis can be completed to assess the annual cost of solar thermal energy.
119
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Appendix A
Solar Array DAQ Wiring Diagrams
128
129
130 Start Read sensor signals (Outlet and Inlet) Is pump on? Is Tout-Tin>TH Is Tout-Tin<TL? Turn pump on No yes
Turn pump off
Is either temperature hotter than Thighlimit Is pump on? yes no Activate S1, S2 and dry cooler
Turn pump on no
yes
yes yes
Are S1, S2 and dry cooler activated? no Is either temperature hotter than Thighlimit yes De-activate S1, S2 and dry cooler no no no yes
Fig. A-4: Flowchart showing solar controller logic Table A-1: Controller temperature parameters
Thighlimit (C) 95
TH (C) 7
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Appendix B
Fortran TRNSYS Algorithms
Conditioner Model (TYPE 251)
1. The vapour pressure of the desiccant at the inlet concentration and cooling water inlet temperature ( was determined using correlations from Conde (2004). was then determined by equating the vapour pressure of air to this desiccant vapour pressure. The outlet humidity ratio was then determined using the Eq. 4.2.6 for dehumidification effectiveness. TL
2. The minimum outlet enthalpy (
)
was determined by calculating the enthalpy of air at the minimum humidity ratio, , and the inlet cooling water temperature. The outletenthalpy of air was then calculated using the enthalpy effectiveness (Eq. 4.2.7). The ASRHAE correlation, Eq. B-1, was used to calculate the temperature of the outlet air.
(B-1)
3. The outlet desiccant temperature Td,out,c was determined from the desiccant cooling water heat
transfer effectiveness (Eq. 4.2.8).
4. The rate of water absorption, ̇ , was calculated from the mass balance on the air stream as given by Eq. 4.2.1. The outlet flow rate of desiccant was calculated from:
̇
̇
̇
(B-2)132
balance between the inlet and outlet conditions as given by Eq. B-3.
̇ ̇ ( (B-3) 6. Using Eq. 4.2.2 and Eq. 4.2.4 ̇ and ̇ were calculated. Then using Equation 3.5, the
outlet cooling water temperature, Tcw,o was calculated.
7. Finally, the outlet concentration was determined from Eq. 4.3.2.
Desiccant Sump Model (TYPE 299)
1. Sump density, concentration, temperature, solid desiccant mass, water mass, and liquid desiccant mass are retrieved from the stored variables array from the end of the previous time step. In the initial time step these values are calculated from the type parameters (Initial volume, initial concentration, initial temperature)
2. The total outlet flow rate based on values from the end of the previous time step is calculated.
3. Eq. 4.4.1 is solved using the DIFFERENTIAL_EQ() function. The mass of liquid desiccant and density from the previous time step are used to calculate a and b for the function. 4. Eq. 4.4.2 is solved using the DIFFERENTIAL_EQ() function. The mass of liquid desiccant
and density from the previous time step are used to calculate a and b for the function. 5. The liquid desiccant mass and concentration are calculated using the solution to Eq. 4.4.1
and Eq. 4.4.2. Both “average” values over the time step and values at the end of the time step are calculated.
6. The energy balance, Eq. 3.4.5, is solved using the new desiccant mass.
7. The average tank density and tank density at the end of the time step are calculated using correlations from Conde (2004).
133
8. The outlet mass flow rates are calculated based on the new densities.
9. Sump density, concentration, temperature, solid desiccant mass, water mass, and liquid desiccant mass from the end of the time step are stored in the storage array to be retrieved in the next time step
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Appendix C
Data Table for Cooling Tower
Table C-1: Performance data for TYPE 51a Air
Volumetric Flow Rate
(m3/hr)
Air Dry Bulb Temperature (°C) Air Wet Bulb Temperature (°C) Water Mass Flow Rate (kg/hr) Water Inlet Temperature (°C) Water Outlet Temperature (°C) 8326 29.5 23 8992.8 35 27.7 8326 32.1 25.6 8992.8 35 30.0 8326 33.5 27 8992.8 35 29.8 8326 29.5 23 7200 35 27.1 8326 32.1 25.6 7200 35 28.5 8326 33.5 27 7200 35 29.4 8326 29.5 23 10800 35 28.3 8326 32.1 25.6 10800 35 29.4 8326 33.5 27 10800 35 30.2
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Appendix D
Data Tables for Biaxial Incidence Angle Modifiers
The evacuated tube solar collector model uses external data tables to estimate the biaxial incidence angle modifier. Table D-1 lists the data table given to TYPE71 (Kingspan, 2012), and Table D-1 lists the data used for collector 2.
Table D- 1: Biaxial incidence angle modifiers for evacuated tube collectors