Mitigation of the energy-water collision through integrated
rooftop solar and water harvesting and use for cooling
Speakers:
Peterson, Eric L1 2
1
2
Civil Engineering, University of Queensland, St Lucia, Australia, e.peterson@uq.edu.au
Institute for Sustainability and Innovation, Victoria University, Melbourne, Australia
Abstract: Conservation policy is usually developed for regions that encompass only one
environmental realm because of logistical, institutional and political constraints. This is
inadequate because these realms often interact through processes that form, utilize and
maintain interfaces or connections, which are essential for the persistence of some species
and ecosystem functions. I present a conceptual framework for sustainable development that
explicitly accounts for energy and water demand of the built environment offset by rooftop
harvest opportunities. I wish to challenge you to think of PV solar and rainwater that can be
captured on a roof and used in evaporative coolers and water-cooled vapour-compression
air-conditioning as a substitute for water and electricity that would otherwise be drawn from
networks. Rainwater cisterns are often overflowing without the recognition that they are full
of refrigerant R718, containing 2.37 kW-hours of "coolth" per US Gallon which can be
applied to improve the energy-efficiency of air-conditioning systems, and to mitigate urban
heat islands by supporting vegetation and water-features. To this end I have developed a
rainwater harvesting wizard http://gettanked.org/ coupled to estimate demand for irrigation,
swimming pools and evaporative coolers. I am contemplating adding a complementary
rooftop solar PV system sizing tool for climates where monsoonal conditions require vapourcompression dehumidification. In the process I have been assisting in the informing
Australian and New Zealand households of the relative efficiency of heat and cooling
appliances that have emerged on the market. So I have prepared design data for 69
Australian and 18 New Zealand locations in collaboration with New Zealand's National
Institute of Water and Atmospheric Research (NIWA) in updates of the climate data files used
in Australian and New Zealand house energy rating schemes (HERS). Design data are
normally distilled from meteorological records to represent design conditions that are
exceeded only rarely, but can also be derived from representative meteorological year (TMY)
files provided at the EnergyPlus website. ASHRAE’s (2013) analysis of World Meteorological
Office database also informs the limits of evaporative cooling effectiveness, but the TMY files
are essential to obtain bins of coincident parameters that measure the frosting of heatpumps.
The design data are temperature, humidity and solar radiation coincident with extreme
events. Design data are required for calculation of peak thermal demand of the built
environment. These data are essential to the design of heating, ventilation and airconditioning (HVAC) systems and will be posted at http://suntank.org/ as they are developed.
Keywords: Rainwater Harvesting System; Evaporative Cooling
1
Why the world needs rainwater for cooling buildings
Coolth is a mass noun of the English language that indicates a pleasantly low temperature, a
status that is the driver behind the air-conditioning industry (Prins 1992). The market to
deliver coolth in the built environment can be to partially served by rooftop harvested
rainwater, similar to the intermitant supply of electricity by rooftop solar panels.
The creation of coolth indoors while it is hot outdoors is analous to the creation of warmth
indoors while it is cold outdoors. The demand to deliver a perception of warmth and coolth is
less critical than the duty of refrigeration in food stores. Refrigeration is measured by the
“ton” capacity (12,000 Btu/hr or 3.51 kW), reflecting the deep cooling effect of constantly
melting 2,200 pounds of ice each day, sourced by an international industry (Blain 2006). The
use of refrigeration capacity “tons” may be misguided in the coolth market, and so the US
Gallon of liquid water is proposed as a measure in keeping with the imperial units. Willis
Carrier (1911) developed an industry founded on maintaining coolth throughout the built
environment – without regard to passive design principles. Air-conditioning, being defined
by the shear well served by water.
Rainwater harvested from a roof and stored for use in evapotranspiration demands of green
roofs, living walls, urban agriculture, and firefighting depends on the 2.26 MJ embodied
coolth per litre of water as it transitions from liquid to vapour. So let us value the coolth of
rainwater harvested, stored, and used in all manor of evaporative cooling processes.
The coolth of liquid water has also been essential to the baseload thermal power industry that
has here-to-for provided 90% of electricity generation (Wiser 2000). Coal fired thermal
power systems typically consume over ½ US gallon per kWh electricity generation (Meldrum
et al 2013), while nuclear and concentrating solar demand substantially more water per
generated kWh. Since the electrity demand of air-conditioning is typically 1:3 (seasonal
average of 3 COP), the indirect water use of air conditioning is ½ US gallon for each 3 kWh
cooling demand. This is comparable to the cooling effect of direct evaporative cooling, as
water contains 2.37 kW-hours of "coolth" per US Gallon. A clear advantage of evaporative
cooling systems are that they avoid the greenhouse gas emmisions associated with coal fired
thermal power, and can be decentralized with rooftop harvesting to serve in-house demand for
cooling. The disadvantage of evaporative cooling is that it does not perform during humid
weather, but in such cases the demand for coolth could be complemented by vapourcompression systems. A building-integrated hybrid air-conditioning system (Williams and
Stockwell 2004) demonstrated solar PV-array powered vapour-compression chilling of the
sump water in conventional evaporative coolers to serve during muggy weather.
If nothing else, the present paper seeks to establish liquid water as a form of renewable energy
which has the capacity to offset a substantial share of the demand for fossil-fuel power
generation. In common with wind or solar energy, rainwater is of intermitant utility.
The demand for coolth delivered by water operates in competion with agricultural and
metropolitian consumption, which have here-to-for resulted in damming of rivers to create
water storages. Because dams are terribly disruptive to landscape and riparian processes, and
suffer from large evaporative losses, it is worth considering alternative water resources.
2
Desalination can recharge evaporating storages, and in the case of solar photovoltaic reverse
osmosis (PV-RO), productivity is maximized in times of drought (Peterson and Gray 2012).
Even in situations where the sea offers the only available water resource, reverse osmosis
with energy recovery requires only 0.01 kWh per US Gallon permeate (2.5 kWh per kL,
Geisler, et al. 2001), which is a small fraction of the coolth potential of the water delivered.
With the advances in rooftop solar electricity, peri-urban and rural developments are feasible
off-grid, without water and power mains. Such developments concurrently require buildingintegrated rooftop rainwater harvesting and storage systems. There has been a long history of
outback Australians dwelling without access to reticulated water, but not without tanks
running dry at times of drought and fire (O’Brien 1921). A novel trans-climatic rainwater
harvesting system design tool, accessed from the URL http://GetTanked.org/, has been
developed (Peterson 2014) after reviewing the literature on the basis of established design
tools, disaggregation of consumption into demand for potable and non-potable uses, and by
assessing variability of rainfall and evapotranspiration across the continent of Australia. The
methodology behind the rainwater harvesting simulations are outlined in Figure 1 includes a
daily mass balance with reference to 121 years of meteorological observations throughout the
Australian continent.
Global hourly monitoring of rainfall has commenced this year with Japan’s launch of a core
observatory satellite now in the process of providing calibration to a constellation of existing
satellites so that their radar can measure light rain (Hou, et al. 2008). This should allow
postprocessing of data from the Tropical Rainfall Measuring Mission, launched in 1997, that
originally only measured moderate and heavy rainfall in the tropics. And so it is concievable
that the rainwater harvesting tool could serve globally with reference to the last 17 years of
rainfall.
Hourly solar energy has been calibrated to provide hourly direct and diffuse timeseries on
virtually an location in Australia since 1990, and these data have been integrated with 80
meteorological ground stations in the Australian Climate Data Bank (NatHERS 2012, Liley,
et al. 2013) as TMY2 files. Perhaps global rainfall datasets can be included in TMY files as
global percipitation is calibrated. Presently very few hourly TMY archives provide unbroken
records of percipitation, and so integrated simulation of rainfall and solar energy impact on
the built environment has not progressed.
3
Figure 1: Rainwater
harvesting and demand system modelled at http://GetTanked.org/
With respect to evaporative cooling, cooling load is calculate on the basis of base 24°C
cooling degree days (CDD) and the design drybulb temperature (DDB) at any locality of
interest. CDD and DDB are determined by the tool, but can also be determined from
ASHRAE (2013), as an independent check. From this point it is assumed that the cooling
rating of the evaporative cooler is suitably sized with respect to DDB, with building heat load,
UA given in equation 1 as the rated cooling capacity kW per Kelvin ∆T.
UA = kW/(DDB - 24°C)
(1)
The displacement airflow of the evaporative cooling system is given in equation 2.
airflow = kW/( ∆T × ρair × Cp)
(2)
Then equations 3 through 6 must be calculated for each day of the simulation.
Cooling demand, kWt = UA × CDDt
(3)
Cooling capacity, kW = airflow × WBCDDt × ρair × Cp
(4)
Delivered evaporative cooling, kWevap = minimum{kWt, kW}
(5)
Water use of evaporative cooler = kWevap × 3600 × 24/(LHE × ρwater)
(6)
Where the latent heat of evaporation, LHE is taken as a constant 2270 kJ/kg water. The
demand of water used in evaporative cooling is thus determined if kW capacity requirement is
known. The forgoing method does not determine the dis-satisfaction that may result from
evaporative cooling, which may be considerable in hot/humid locaties. So it is necessary to
identify such problem areas with the red overlay circles in Figure 2. The circles of this figure
are classified by the kW capacity of an 85% effective direct evaporative cooler with 10³/hr
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air-flow, where the red circles denote locations with less than 1 ton capacity (3.5 kW). The
map shows localities where evaporative cooling is effective, with over 22 kW capacity (blue
circles) in colder climates of Tasmania and the Austrlian Alps. The area of application is
more promising around the orange, yellow, and green circles but depends on the availablity of
water with reference to the shades of grey that classify sustainable yield from cistern tanks.
Figure 2: Direct evaporative cooler capacity plotted over grey scale simulated
sustainable daily demand that can be delivered per m² catchment per hL cistern storage
tank capacity. Circles are from locations of ASHRAE design data. Polygons represent
regional performance based only on the assigned meteorological station of each
NatHERS climate zone.
This map does not present the demand for cooling, which requires a detailed building thermal
performance analysis. New homes that comply with the building code of Australia would be
well served with 100 W/m² cooling capacity, or about 3 tons serving 100 m² dwelling. This
map marks a red circle in locations where evaporative is ineffective due to humidity, and
white in regions where rainwater harvesting systems would fail during dry seasons or drought.
HVAC system efficiency can be improved by exploiting the annals of the Australian Climate
Data Bank (ACDB) that documents climate variability at 80 locations around Australia from
5
1967 through 2012. In contrast, mid-20th-century design temperatures are listed in Table 1 of
the AIRAH Handbook “DA09 Air Conditioning Load Estimation Application Manual”.
DA9 Table 1 includes 60 hourly stations in Australia from which HVAC systems can be
designed to suit critical conditions, while synthetic data was offered in Appendix Table 1B to
cover 600 locations. DA9 Table 1 design conditions are generally based on 10 years of
hourly observations, excluding meteorological data subsequent to the Australian Bicentennial.
Australian HVAC system design data website http://uq.id.au/e.peterson/ now increases the
temporal coverage of HVAC design data by exploiting the ACDB, being a set of 80 high
quality annals of hourly meteorological data that generally span 45 years (1967-2012)
developed by Liley et al. (2013). This provides HVAC designers with design data to detail the
current climate at 80 key locations, and track local changes with respect to the past 45 years in
order to inform likely conditions mid-century-21. The methods presented here can be adapted
to increase spatial coverage to 600 locations by working with the Bureau of Meteorology to
process automatic weather station data.
In other cases, the Queensland Government’s SILO database of thousands of rainguages
instrumented with daily minimum and maximum temperature records can be used to estimate
HVAC design conditions. This is delivered from the rainwater harvesting and demand
calculator that has been developed at http://gettanked.org/ and by selecting “evaporative
cooling” and specifying a nominal capacity. The use of daily maximum observations to
estimate design conditions was detailed by Peterson et al. (2006).
DA9 includes 14 hourly stations that are not covered by the ACDB because they were closed
manual stations. Consequently we will need to process Bureau of Meteorology automatic
weather station data near the older manned stations to complete revision of Table 1 at these
locations. A case study of urban heat island effects in the Brisbane capital city central
business district illustrates the issues.
Conclusions
Rooftop solar PV systems may be required for climates which require vapour-compression
dehumidification. Australian and New Zealand households’ heating and cooling demands are
well placed relative to the efficiency of heat pumps in analysis of the climate data files used in
Australian and New Zealand house energy rating schemes (HERS). Design data are distilled
from meteorological records to represent design conditions that are exceeded only rarely, but
can also be derived from representative meteorological year (TMY) files. ASHRAE’s (2013)
analysis of World Meteorological Office database also informs the limits of evaporative
cooling effectiveness, while TMY files provide bins of coincident parameters that measure
the frosting of heatpumps. The design data are temperature, humidity and solar radiation
coincident with extreme events. Design data are required for calculation of peak thermal
demand of the built environment. to the HVAC systems and will be posted at
http://suntank.org/ as they are developed.
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References
ASHRAE 2013. Fundamentals. American Society of Heating, Refrigeration, and Air-conditioning
Engineers, Atlanta.
Blain, Bodil Bjerkvik (February 2006). Melting Markets: The Rise and Decline of the AngloNorwegian Ice Trade, 1850–1920. Working Papers of the Global Economic History Network. 20/06.
London: London School of Economics.
Carrier, W. H. (1911). Rational Psychrometric Formulae: Their Relation to the Problems of
Meteorology and of Air Conditioning. No. 1340. Presented at the annual meeting 1911, of the
American Society of Mechanical Engineers. p. 1005-1053
Geisler, P, Krumm, W, and Peters, TA (2001). Reduction of the energy demand for seawater RO with
the pressure exchange systems PES. Desalination 135:205-210.
Hou, A. Y., Skofronick-Jackson, G., Kummerow, C. D., & Shepherd, J. M. (2008). Global
precipitation measurement. In Precipitation: advances in measurement, estimation and prediction (pp.
131-169). Springer Berlin Heidelberg.
Liley, J.B.,Nagle, P. and Grant, I. (2013). “Australian climate data for building energy simulation”,
AIRAH Building Simulation Workshop Melbourne April 18, 2013.
Meldrum J, Nettles-Anderson S, Heath G, and Macknick J (2013). Life cycle water use for electricity
generation: a review and harmonization of literature estimates. Environ. Res. Lett.
NatHERS (2012). The Commonwealth of Australia as represented by the Department of Industry (the
Commonwealth) has developed Nationwide House Energy Rating Scheme (NatHERS) climate files
2012 compiled from Bureau of Meteorology (BOM) raw climate data and then used to complete a
typical year for every NatHERS climate zone in Australia.
O’Brien, JP (P J Hartigan) (1921). Said Hanrahan In: Around the Boree Log & Other Verses.
Peterson, E.L.,Williams, N., Gilbert, D., and Bremhost, K. (2006). New air conditioning design
temperatures for Queensland, Australia, AIRAH Ecolibrium, Vol. 5, No. 1, pp.26-36
Peterson, E. L. and Gray, S. (2012). Effectiveness of desalination powered by a tracking solar array to
treat saline bore water. Desalination 293:94-103.
Peterson, EL (2014) Transclimatic simulation tool to design secure rainwater harvesting systems
across Australia. Under Review RECYCL-D-14-00275 Resources, Conservation & Recycling.
Prins, G. (1992). On condis and coolth. Energy and Buildings, 18(3), 251-258.
Williams, N.W. and Stockwell, B.V. (2004) Air conditioning system. EP 1787065 A1 Patent filed by
the State of Queensland (2007).
Wiser, Wendell H. (2000). Energy resources: occurrence, production, conversion, use. Birkhäuser. p.
190.
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Glossary
ACDB
Australian Climatic Data Bank, solar radiation and surface meteorology
AIRAH
Australian Institute of Refrigeration, Air Conditioning and Heating
ASHRAE
American Society of Heating, Refrigerating and Air Conditioning Engineers
CDDt
Cooling degree days with reference to base temperature t (generally 24 ° C)
Coolth
pleasantly low temperature
COP
coefficient of performance, ratio of heating or cooling delivered to electricity demand
DA
design aid, AIRAH technical manual for HVAC and refrigeration design
DDB
HVAC system design dry bulb temperature
HERS
house energy rating scheme, relative score of a residential dwelling’s energy demand
HVAC
heating, ventilation, and air conditioning, technology of indoor environmental control
LHE
latent heat of evaporation, enthalpy to transform a substance from liquid to vapour
NatHERS
Nationwide House Energy Rating Scheme, Australian dwelling thermal performance
NIWA
National Institute of Water and Atmospheric Research, Lauder, New Zealand
PV
photovoltaic conversion of solar radiation into electricity using semiconductors
RO
reverse osmosis, water purification technology that uses a semipermeable membrane
R718
water used as the evaporating fluid in a refrigeration cycle
SILO
daily database of Australian climate data from 1889
TR
ton of refrigeration, heat-extraction capacity by melting 0.907 tonne of ice per day
TMY
typical meteorological year, collation of hourly weather data for a specific
location, generated from a data bank of many years duration. Consituent months are selected
to presents the range of weather phenomena for the location in question, while still giving
annual averages that are consistent with the long-term averages for the location in question.
UA
overall building fabric demand for cooling per increment outdoor temperature
US Gallon
customary unit of liquid capacity of the United States, 3.78541 litres
8