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Raw materials Transport & Construction Maintenance Operation Disposal Embodied energy and carbon in materials Fuel Electricity Equipment’s consumables Electricity MW Fuel GHG GHG Replaced consumables GHG GHG Construction waste WW RW RHO_mx: A Life-cycle Comparative Model for Rainwater Harvesting MSc Environmental Systems Engineering María del Carmen Valdez Berriozábal Supervisors: Ilan Adler and Mark Barrett Introduction RHO_mx is a comparative simulation model that quantifies and compares total greenhouse gases (GHG) emissions of buildings supplied by the municipal grid in Mexico City against different configurations of rainwater harvesting (RWH) systems. Their potential to reduce rainwater poured into the drainage system is also compared with the objective of acknowledging RWH systems’ potential to mitigate flooding risk. Context Mexico City has particular characteristics that make RWH convenient: • It imports 18% of its water demand from other hydrologic basins because its aquifer is overexploited and has caused land subsidence. • Consumes 1.23 kWh/m 3 to supply water to the city, 65% of it for importing water by pumping it over 1.1km high mountains. • Water-network leakages are estimated to be 49% of water supplied. • It is prone to flood for being located in an endorheic basin and having its natural infiltration areas urbanised. • Subsidence has increased flooding risk and made necessary to pump wastewater and rainwater mixed with it. • Buildings have underground water storages and pump water to header tanks because of grid’s lack of pressure due to its leakages. Methodology Results Scenario 1. More than 93% of the GHG emitted by buildings supplied only by municipal-water in Mexico City occurs during its operation. From this, more than 77% is caused by municipal-water consumption. -450.0 -350.0 -250.0 -150.0 -50.0 50.0 150.0 4 5 4 5 4 5 4 5 4 5 4 5 4 5 4 5 4 5 4 5 4 5 House Ceylán Culiacán S. MarinaAmsterdam Eugenia Office 2s Office 5s Office 10s Retail 1s Retail 5s kg CO2eq/yr Raw materials Transporta on Construc. & Maintenance Opera on Disposal Preliminary conclusions. Scenario 4 achieves the highest GWP reductions and retains storm-water. It is not economically viable for dwellings with low water-bills- savings/initial-investment ratio. Scenario 5 is recommended for high-rise buildings where rainwater infiltration by natural means is feasible. Scenario 1 All uses supplied by MW grid. MW is stored in a PW cistern and then pumped to header tanks. RW from roof is discharged to sewer. Scenario 2 Independent RW and PW cisterns for independent uses and distribution systems with pumps and header tanks. The methodology used is life-cycle assessment. Inputs and outputs through the different processes are analysed. Impact indicators measured are: global warming potential (GWP) and cumulative energy demand (CED). Scenarios Five scenarios are analysed in twelve different types of buildings. Scenario 1 is the base case with no RWH. Scenario 2 is a common practice. Rainwater storage size. Enlarging the size calculated by RHO_mx to cope with dry years has little effect in GWP and CED if it is located in the flat part of their curves. But cost will increase unless high water tariffs are being paid. Two-storey office example: Scenario 3 Independent RW and PW cisterns, but RW is treated and distributed with potable-water using the same pumps and header tanks. Scenario 5 Several RW wall-mounted tanks in different levels of the building harvest rain for non potable uses. MW is supplied as in base case with pumps. Scenario 4 One cistern for RW and PW mixed, all water is treated and distributed together using pumps and header tanks. 529 7,245 28,581 7,542 4,764 3,970 7,245 959 3,705 7,410 2,223 11,115 161 2,147 8,755 2,260 1,473 1,179 2,428 292 1,153 2,792 638 3,303 135 472 2,508 614 690 334 581 220 503 856 269 711 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Single-house H-Ceylán H-Uxmal #250 H-Culiacán #26 H-Secretaría de Marina #500 H-Amsterdam #42 H-Eugenia #1009 Two-storey office Five-storey office Ten-storey office One-storey retail Five-storey retail kWh/yr Opera on: MW supply Opera on: others Raw materials Transporta on Construc. & Maintenance Disposal Energy consumption by life-cycle process GHG emissions by life-cycle process house 0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 20% 0% 5% 10% 15% 20% 25% 30% GWP reduc on Ra o available-rainwater/water-demand 1 2 3 4 5 Poli Poli Poli 0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 20% Single-house H-Ceylán H-Uxmal #250 H-Culiacán #26 H-Secretaría de Marina #500 H-Amsterdam #42 H-Eugenia #1009 Two-storey office Five-storey office Ten-storey office One-storey retail Five-storey retail GWP reduc on Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 GWP reduction by scenario and building GHG increments and reductions for scenarios 4 & 5 Potential GWP reduction by scenario and ratio RW/WD scenario 4: probability of exceeding target harvesting volume scenario 4: GWP, CED and cost variation by cistern size 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 -15.00 -10.00 -5.00 0.00 5.00 10.00 15.00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 % kgCO2eq/m3 , kWh/m3 ,£/m3 Volume of the rainwater cistern or total reten on tanks [m3] Selected size Global Warming Poten al (GWP) [kgCO2e/m3] Cumula ve Energy Demand (CED) [kWh/m3] Cost [£/m3] RW frac on from water demand [%] September 2014 213.94 99.48 153.65 1.311 1.584 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 50 100 150 200 250 3% 10% 16% 23% 29% 35% 42% 48% 55% 61% 68% 74% 81% 87% 94% 100% Kg CO2e/m3 m3/yr Rainwater harvested 16.79m3 deposit [m3/yr] Target RW harvested [m3/yr] Resul ng GWP [Kg CO2e/m3] GWP in scenario 1 Storeys Units Water demand (WD) [m3/yr] Available RW fraction of WD [m3/yr] Non- potable water demand [m3/yr] Lowest GWP scenario Building Single-house 2 1 292 30.0% 33% 4 H-Ceylán 5 20 3,997 3.5% 33% 5 H-Uxmal #250 3 76 15,768 9.2% 33% 4 H-Culiacán #26 5 19 4,161 5.4% 33% 5 H-Secretaría de Marina #500 3 12 2,628 10.9% 33% 4 H-Amsterdam #42 7 10 2,190 3.8% 33% 5 H-Eugenia #1009 5 20 3,997 3.9% 33% 5 Two-storey office 2 1 529 29.7% 45% 4 Five-storey office 5 1 2,044 12.0% 45% 5 Ten-storey office 10 1 4,088 6.0% 45% 5 One-storey retail 1 1 1,226 20.0% 45% 4 Five-storey retail 5 1 6,132 4.0% 45% 5 Constant 4 5 4 5 29.2% 12.7% 9.3 0.7 3.5% 3.3% 44.0 3.2 9.2% 4.0% 244.2 6.1 5.4% 4.8% 52.1 4.0 10.9% 7.2% 44.2 2.5 3.8% 3.7% 24.6 3.0 3.9% 3.8% 45.3 4.7 29.0% 14.1% 16.8 1.1 12.0% 9.4% 36.2 4.0 8.0% 5.2% 31.8 1.4 19.9% 6.5% 29.4 0.7 4.0% 3.5% 69.8 4.0 Supplied RW fraction of WD Rainwater storage size [m3] Minimum size for storms [m3] Highest NPV/m3 scenario Water tariff [£/m3] Ratio water- bill-savings / initial cost Constant Constant 4 5 4 6.91 5 -£1.21 -£1.47 £6.91 -£0.06 11.2 5 -£1.21 £0.55 £1.81 -£0.08 114.61 4 -£1.21 £4.66 £2.31 -£0.18 17.86 5 -£1.21 £1.65 £2.64 -£0.11 22.64 5 -£1.21 £3.92 £4.05 -£0.12 6.65 5 -£1.21 -£0.33 £1.83 -£0.06 12.44 5 -£1.21 £0.77 £1.99 -£0.09 12.44 4 -£1.60 £11.90 £10.74 -£0.12 19.36 4 -£3.31 £16.46 £15.02 -£0.31 13.83 5 -£3.31 £8.16 £9.03 -£0.31 19.36 4 -£3.31 £27.35 £10.59 -£0.31 19.36 5 -£3.31 £5.39 £5.52 -£0.30 NPV per unit [£/m3]
