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Aspects of solar thermal hot water hea0ng
Professor Tim Dwyer [email protected]
Teaching Fellow, UCL
CIBSE South Wales – 2 April 2012
Presentation available for download at www.timdwyer.com/presentations/20120402cardiffsolar.pdf
30 min of solar radia0on falling on earth is probably equal to the world energy demand for one year A Researcher
Why more UK solar thermal? • almost no emissions
– some related to pumping/controls • life of 20 to 30 years • independence from fuel price infla0on • total cost analysis based on known cost • low maintenance • poten0al for government subsidies • certainty of fuel supply • environmental cred!!
Solar Trade Association, 2007
Monthly solar irradiance (kWh) on a flat plane facing South with a tilt angle of 45° Data BS EN 15316-4-3:2007
Solar Data for Coryton
Average irradiation kWh/m²·day
Source: http://re.jrc.ec.europa.eu/pvgis
Annual H - 1022 kWh/m² N - 1172 kWh/m² V – 821 kWh/m²
0
1
2
3
4
5
6
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Horizontal
Normal
Ver0cal
Source:CIBSE Solar Heating Design and Installation Guide
Viability of Solar Thermal Systems Amount of annual sunshine Required temperature of hot water
Annual energy requirement and energy use profile
Capital cost of the solar system
Prices of conven0onal fuels
Fuel price infla0on
Solar system annual O&M cost
Other (e.g. legisla0on, government funding)
Typical Bands of Solar Thermal
Low Temperature (100°C)
Industrial process hea0ng Electricity genera0on
Solar thermal and PV working together
Collec0ng the heat
• Flat plate collectors – Unglazed – Glazed
• Evacuated tubes – Direct flow – Heat pipe
Unglazed Collector • Low-‐cost unglazed collectors
• Summer pools in cold climates
• Extend the season in warm climates
• For summer use on a year-‐round pool in cold climates
• Can payback in few years
Flat Plate Glazed Collector
Solar Thermal Systems, Peuser et al 2002
Integrated
Solar Thermal Systems, Peuser et al 2002
Retrofieed
hep://www.solar-‐ra0ng.org/educa0on/criteria/collector/6_5_12.htm
Framed installa0ons
But what area?
Solar Thermal Systems, Peuser et al 2002
Evacuated tubes
Sydney Tube
Direct Flow
Solar Thermal Systems, Peuser et al 2002
Solar Thermal Systems, Peuser et al 2002
Solar Thermal Systems, Peuser et al 2002
Heat Pipe
http://www.reuk.co.uk/Evacuated-Tube-Solar-Water-Heating.htm http://www.solarpanelsplus.com/thermal-how-it-works/
Heat Pipe Collector
http://www.solarpanelsplus.com/thermal-how-it-works/
What area?
Defining and cer0fying performance
ASHRAE 93 -‐ Methods of Tes0ng to Determine Thermal Performance of Solar Collectors
EN12975-‐2 Thermal Solar Systems and Components
CIBSE KS15 Capturing Solar Energy 2009
Conversion factor or Optical Efficiency
Heat Loss = Useful Gain
ASHRAE 93
EN12975-2
1.74 m2 aperture 1.78 m2 absorber
Cer0fica0on
Source: Kingspan Thermomax Design Guide
12975 Efficiency Example
A panel shown earlier has the following characteris0cs supplied by the test Efficiency at Δt of 0, η0 = 0.814
Loss Coefficient: a1 = 4.954 W/(m2K) Loss Coefficient: a2 = 0.0189 W/(m2K2) Ambient temperature = 20°C Average water temp [(tin+ tout)/2] = 70°C
Global irradiance of 800waes/m2
Solu0on to Example
Using η = η0 -‐ (a1 x (tm – ta)/G) -‐ a2 x (tm – ta)2/G)
Solu0on to Example
Using η = η0 -‐ (a1 x (tm – ta)/G) -‐ a2 x (tm – ta)2/G)
η = 0.814 -‐ (4.954 x (70 – 20)/800)
-‐ 0.0189 x (70 – 20)2/800)
Solu0on to Example
Using η = η0 -‐ (a1 x (tm – ta)/G) -‐ a2 x (tm – ta)2/G)
η = 0.814 -‐ (4.954 x (70 – 20)/800)
-‐ 0.0189 x (70 – 20)2/800)
= 0.814 – 0.3096 – 0.0024 = 0.502
ie at these condi0ons η= 50%
50% of the energy provided by the sun is actually used to heat the water
RETScreen
RETScreen
CIBSE KS15 Capturing Solar Energy 2009
Key Types • Passive Systems (no pumps)
– Integral Collector Storage – Thermosyphon
• Ac0ve Systems (pumps & controls) …. • Open Loop:
– Direct – Drain Down
• Closed Loop: – Drain Back – An0freeze
Integrated Collector and Storage
Novel ICS System
Parabolic collector water heater
Thermosyphon System
Solar thermal collectors and applications Soteris A. Kalogirou
http://picasaweb.google.com/bertmenkveld/REDWHO2008SolarThermal#
http://www.p2pays.org/ref/20/19122/HeatCool.html
Thermosyphon Unit
www.solarshacksa.com/html/hot_water.html
Direct Circula0on System
Solar Hea0ng Guide – CIBSE 2007
Drainback System
CORRECTED DIAGRAM BASED ON Solar thermal collectors and applications Soteris A. Kalogirou
Other drainback (plus other system) illustrations http://greenterrafirma.com/solar_thermal.html http://www.atlassolarinnovations.com/solar-water-heating-choices/
EST CE131 - Solar water heating systems – guidance for professionals, conventional indirect models
Drain Back Advantages
• Gravity is fail-‐proof and maintenance free • Water, (or a glycol mixture) may be used in the collector loop
• System is not damaged if the pump fails • System cannot reverse thermosyphon at night • Collector plates last up to longer than in a pressurized glycol system
Drain Back Advantages
• If used the Water/Glycol Mixture is unlikely to need changing
• Less moving components to fail (eg. check-‐valves, air vents and expansion tanks)
• Collector piping and system piping does not scale if dis0lled water is used.
Drain Back Disadvantages
• Collector(s) and all piping must be above and slope downwards towards the reservoir
• Larger piping and insula0on must be used • Large or rela0vely large pumps
– especially if the design involves 2+ stories • System and pump controls cost approx 10% of savings
• Components cost about 10-‐15% more than a glycol system (on residen0al system)
• Systems can be noisy -‐ like a coffee percolator
Indirect Closed Water System
Solar Hea0ng Guide – CIBSE 2007
Simplified - No safety/expansion elements shown
A common closed system
Source: Megaflo
Solar Hea0ng Guide – CIBSE 2007
EST CE131 - Solar water heating systems – guidance for professionals, conventional indirect models
www.wagner-‐solar.com
Solar Hea0ng Guide – CIBSE 2007
Solar Hea0ng Guide – CIBSE 2007
Likely losses?
Source: EST 131, 2006
Effect of parasi0c losses (shown in terms of primary energy)
Note: tests undertaken in 2001
Source: DTI/Pub URN 01/1292
Well performing domes0c system
Energy Savings Trust: Here comes the sun: a field trial of solar water heating systems
Aeributes of the Heat Transfer Fluid
• Should not deposit …… – limescale – sludge, – ice – other solids
……that could restrict circula0on or impair the rate of heat transfer.
Aeributes of the Heat Transfer Fluid
Resist freezing
Specially where the liquids are maintained in loop all year round
Usually 60/40 water/an0freeze
will go down to at least -‐20 at SAP Glycol ‘creeps’ more than water
Use non-‐toxic propylene glycol Glycol not compa0ble with Zinc or certain seals
Most ‘an0 freezes’ will have higher viscosity and small SHC
pressure drop of the system will increase
Aeributes of the Heat Transfer Fluid
Maintain integrity at high temperatures • during periods of stagnancy collector temperatures can reach 200°C in flat plates and 300°C in evacuated tubes
• azer a shutdown other parts of system may well reach over 160°C+
• in normal opera0on 120°C would not be uncommon
Glycol Vaporisa0on Temperatures
See Duffie & Beckman, 2nd Edition Appendix E for more detailed data
Solar Thermal Systems, Peuser et al 2002
Aeributes of the Heat Transfer Fluid
• Beeer to run at lower pressures and design for vaporisa0on • At 300kPa vaporisa0on at 140°C • Few molecules of fluid would be in high temperature collector
as a vapour – par0cularly where long periods of stagnancy
• May lead to par0cles s0cking to flow channels – use strainers to remove par0culates
• Small collector volume is best with flow and return connec0ons at top – vaporised fluid will fill collector and not pass large amounts of vapour
into remainder of system • at 150°C, 300kPa only 5cm3 liquid is needed to fill 3 litre collector)
Aeributes of the Heat Transfer Fluid
• Important to help prevent heat stress by unevaporated fluids entering the stagnant collector eg for low lying or interconnected units.
• Normal opera0ng condi0ons above 220°C need special fluids Unsuitable fluids chemically crack at this temperature
• The fluid will need replacing – Ageing will affect corrosion resistance and freezing point
Stagna0on Temperature
tstg = stagna0on temperature °C tas = selected ambient temperature °C
Gs = selected solar irradiance W/m2
Gm = solar irradiance (from test data) W/m2
tsm = absorber temperature °C (from test data)
tam = ambient air temperature °C (from test) BS 12975 2001 Capturing Solar Energy 2009
Assumes ratio (tsm-tm)/Gm approx constant
Source: Kingspan Thermomax Design Guide
Antifreeze that has been at 170°C for extended periods
Exacerba0ng stagna0on
Source : Stagnation behaviour of solar systems – IEA SHC Group 26, 2002
Poor layout
Heat dissipa0on
Source: Kingspan Thermomax Design Guide
And don’t forget …..Legionella control
Source : Lochinvar Solar Thermal Guide
But how to choose system?
• Es0mate Daily Water Hea0ng Load • Determine Solar Resource • Calculate Solar System Size
– meet load on sunniest day – undersize rather than oversize
• Calculate Annual Energy Savings • Calculate Annual Cost Savings • Es0mate System Cost • Calculate Payback Period
Where to posi0on?
Source: Kingspan Thermomax Design Guide
Seasonal changes
Source: Kingspan Thermomax Design Guide
Avoid shading
Source: Kingspan Thermomax Design Guide
F-‐Chart
Polysun 4
T Sol
RETScreen
Matched system
Source: Kingspan Thermomax Design Guide
Source: http://www.pobsolar.co.uk/
Bri0sh Standards
BS EN 12975-‐1 (2006)
Thermal solar systems and components — Solar collectors — Part 1: General requirements
BS EN 12975-‐2 2006
Thermal solar systems and components — Solar collectors — Part 2: Test methods
BS 5918 (1989)
Solar hea0ng systems for domes0c hot water
References
ASHRAE (2008) HVAC Systems and Equipment – Chapter 36: Solar Energy Equipment
ASHRAE (2007) Applica0ons Handbook – Chapter 33: Solar Energy Use
Solar water heating systems – guidance for professionals, conventional indirect models Energy Savings Trust CE 131
Energy Savings Trust - Here comes the sun
CIBSE – Solar Heating – Design and Installation Guide
CIBSE – KS15 Capturing solar
energy
Solar Engineering of Thermal Processes, Duffie, & Beckman
Solar Design: Components, Systems, Economics by Jan F. Kreider
Solar Thermal Systems - Successful Planning and Construction Peuser, Remmers & Schnauss
MCS - Microgeneration Installation Standard: MIS 3001
Thanks for providing informa0on…
• Lochinvar • Baxi • Kingspan • John O’Brien • Viessmann
…..and to Ruskin Air Management for sponsoring the event
Prof Tim Dwyer, [email protected]
Where next …..solar Cooling
Stoecker - Refrigeration and Air Conditioning