Solar Thermal Cooling

Solar Cooling
Methods and
Applications
©Sargon Ishaya, PE, LEED AP
Pragmatic PE, Incorporated
408-813-2970

Objectives
Help engineers understanding when and
how to apply solar cooling systems

Describe two practical methods for solar
cooling

Give air conditioning engineers the
confidence to offer customers a
mechanical approach to solar power

Presentation Agenda
Introduction
Load Calculation Significance and Methods
Overview on Applying Green Technologies
PART ONE: Introduction to Solar Cooling (CST vs PV)
Photo-voltaic systems
Solar thermal systems
Efficiencies of solar cooling systems
Compare and contrast PV to CST in Solar Cooling
PART TWO: Application of Solar Cooling (CST or PV)
Where does solar power fit? (The Solar Multiple)
Solar Cooling Nuances
Utilization and the Solar Multiple
Storage and the Solar Multiple

Conclusions
Questions

Load Calculation Methods

 What are the steps to sizing an air conditioning system for a
building?
 Obtain architectural plans
 Write or review Basis of Design and Owner Project Requirements
 Enter space diagnostics into a computer program
 Find peak loads for cooling and heating from computer program
 Size equipment to satisfy the peak loads
 Is the process different when applying technologies for a green
building?
 The answer is a big YES, but why?
 Let’s look at rainwater harvesting in Los Angeles as an example.

 Given information for Rainwater Harvesting Example:
Building is in Los Angeles with monthly precipitation as shown below
 City water is available, but owner would like to be green and harvest rain
 Load is constant and equivalent to 150-mm precipitation on capture surface
 Goal is to minimize cost and complexity of rainwater harvesting
 It should also be said that this graph is suspect, but will suffice for our example

Chart copied from http://www.weather-and-climate.com/average-monthly-precipitation-Rainfall,Los-Angeles,United-States-of-America

 Question: What capacity should the rainwater harvest system have
in terms of mm precipitation per month?
 Peak: The July value so it can be stored during summer for use in winter. This
is analogous to how an HVAC system would be sized.
 150-mm: The capacity needs to match the load.
 Minimum: The January value and use city water for the remaining.
 Answer: It depends on cost, but the minimum (January value) will
most likely be the correct answer.

Chart copied from http://www.weather-and-climate.com/average-monthly-precipitation-Rainfall,Los-Angeles,United-States-of-America

 Lesson: Load calculations regarding green technologies depend on
time of use; therefore, calculations need to be run that way.
 In almost all building and green technology applications the time
step needs to be hourly.
 Example for a wind turbine application in Decatur, Illinois:
 Go to http://rredc.nrel.gov/solar/old_data/nsrdb/1991-
2005/tmy3/by_state_and_city.html and scroll down to Decatur
 Download the file and open it in Excel
 Note the fields at the top and the dates down the side
For a full explanation of all the fields and how to use the data, download the
manual from http://www.nrel.gov/docs/fy08osti/43156.pdf
 Go to column AU and note that wind speed is given for every hour of the
year

 Choose a sample wind turbine. The one I have chosen has the
following characteristics
 Cuts in at 3.5-m/s wind speed
 Is rated at 13-m/s wind speed
 Cuts out at 25-m/s wind speed
 Looking at the Power Output vs. Wind Speed curve of the turbine I
surmise that I’m not going to count a usable hour of wind turbine
output until the wind speed is 7-m/s

 Now go to the spreadsheet and create a toggle column to count
the hours of usable wind per year in Decatur
 On a real job the curve below would be input so the actual power output
would get calculated by the spreadsheet
 The time of day energy use would also be input so that the value of the
power offset by the turbine would be accurately calculated
 After the toggle formula is created, count the hours per year that
the turbine provides usable power
 Answer: 1,375 hours/year
 When do these hours occur?

 Applying solar thermal technologies is not different than the
rainwater harvesting or wind turbine examples
 Load calculations need to be done on an hourly basis to determine if the
application is even worthwhile
 The analysis must optimize the size of the system based on hourly usage and
not peak loads
 Equipment limitations must be taken into account; for example, hot water
tanks cannot store water at temperature overnight
 Spreadsheets are an excellent tool for these analyses and allow for
repeatability and speed
 Money talks so put this dimension into the engineering calculations

Applying Green Technology – Base and Transient Loads
Constant (base) loads are building loads that are not a function of
the time of day nor seasons of the year
Lobby and corridor lighting within a hotel is one example of a hotels’ constant
load components
What are other examples of constant HVAC loads in a building or facility?

Transient Load
Total area under the curve
represents kW-hr per day

Constant (Base) Load

Chart copied from http://www.esource.com/files/esource/images/CEA-06_2F.gif

Transient loads are building loads that are a function of the time of
day or seasons of the year
A fancy restaurant within a hotel is one example of a hotel’s transient load
components
Can you think of any other examples of transient loads in a building or facility?

Transient Load



 Is it possible to engineer transient loads so that they become base
loads? How?

Transient Load



Sometimes it is advantageous to flatten out transient loads so that
they act like base loads (fuel cell applications are like this)
Going back to the example of a hotel, look at the hot water loads
and note that they are mostly transient:
 Showers and lavatory use
 Pool and/or hot tub heating
 Kitchen and Dining Facility - human consumption
 Kitchen and Dining Facility - cleaning
 Laundry
 Space heating
 Is it possible to flatten out these transient loads?

 Using Storage and Time-of-Use Scheduling can make transient
loads base loads, but the operators must comply
Morning Time 12:00 AM 1:00 AM 2:00 AM 3:00 AM 4:00 AM 5:00 AM 6:00 AM 7:00 AM 8:00 AM 9:00 AM 10:00 AM 11:00 AM

Showers/Lavatory Storage Storage Storage Storage

Pool/Hot Tub Boiler Boiler Boiler

Dining/Drinking Boiler Boiler Boiler Boiler Boiler

Kitchen Cleaning Boiler Boiler Boiler Boiler

Laundry Boiler Boiler Boiler

Space Heating Boiler Boiler Boiler Boiler Boiler Boiler Boiler Boiler Boiler

Afternoon Time 12:00 PM 1:00 PM 2:00 PM 3:00 PM 4:00 PM 5:00 PM 6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 11:00 PM

Showers/Lavatory Storage Storage Storage

Pool/Hot Tub Boiler Boiler Boiler Boiler Boiler

Dining/Drinking Boiler Boiler Boiler Boiler Boiler Boiler Boiler Boiler Boiler

Kitchen Cleaning Boiler Boiler Boiler

Laundry

Space Heating Boiler Boiler Boiler Boiler Boiler Boiler Boiler

Applying Green Technology – Correlation and Payback
Payback (or higher Net Present Value) of a green technology is
optimized when the green commodity’s availability and it’s
consumption are correlated (directly proportional to each other)
 Example: Daylighting by which windows and skylights are used in
place of electrical lighting
Daylighting an elementary school classroom (only open during
school hours)
Commodity = sunlight
Consumption = lighting requirement for the room
Are consumption (occupied room) and availability (sunlight) correlated?
Does the electrical infrastructure to fully light the room at night need to be installed?

Applying Green Technology – Correlation and Payback
optimized when the green commodity’s availability and it’s
consumption are correlated (directly proportional to each other)
Daylighting a movie theater
Are consumption (lighting theater between movies) and availability (sunlight)
correlated?
Does the electrical infrastructure to fully light the room at night need to be installed?
Sharing examples of correlated and uncorrelated green
technologies versus loads
 Many well-engineered daylighting systems don’t show substantial
savings. Why do you think this happens?
 In my calculations I don’t factor in the human component of
ignoring the design constraints.

Applying Green Technology – Utilization and Payback
optimized when the most expensive components of the system are
operating 100% of the time
Obvious Example: Company electric vehicles
Vehicles and charging stations are the most expensive components (as opposed to
parking spaces, maintenance, and management)
Electric vehicles save about $0.065 per mile1
If an outside sales engineer drives an average of 100-miles per week and an inside
sales engineer drives 50-miles per week, then who should get the electric vehicle?

Not so Obvious Example: The solar arrays in solar-thermal power
plants
The steam-to-electricity generation system is much, much more expensive than the
solar array (parabolic troughs)
These plants may have a solar array capable of 2-MW when the steam/electrical
infrastructure only handles 1-MW because it makes economic cents/sense

Ramification: Sizing a green system often depends on costs instead
of loads
Typically air conditioning systems are sized to handle the maximum load, but they
operate at about 60% of capacity on average (60% utilization)
A green technology system should not be sized this way; rather, it should be sized so
that 100% of the expensive components are utilized while the system operates
 This is similar to the rainwater harvesting example spoken about earlier
 This is why spreadsheets with hourly calculations and integrated
costs are so important when engineering green technologies like solar
cooling

Examples of Offsetting Base and Transient Loads
with Green Technologies
 Fuel Cells
Typically use natural gas in an emission-free, non combustion process to produce
electricity and high grade waste heat that can be used for heating or cooling
Initial cost of fuel cell is very high compared to other components
What is the green commodity and what is it’s availability?
From a correlation/utilization standpoint, are fuel cells best for constant loads or
transient loads?

Fuel cell in
Nebraska

Examples of Offsetting Base and Transient Loads
with Green Technologies

Wind Turbines (non-utility)
Residential or community wind turbine plants typically
generate more electricity in the evenings
Given this information, what kind of building load would wind
turbines be best suited for?
Note how loads are “targeted” when applying
green technologies

What Happens if Correlation is not possible?
Do commodity availability and consumption have to be correlated?
What can be added to a system to account for uncorrelated consumption and
availability?
Rainwater
Harvesting System
Storage Tank

Electric/Hybrid
Automobile
Battery

Conclusion: Understanding the application and the characteristics of a specific
green technology are very important – NOW LET’S LOOK AT SOLAR COOLING

Solar Cooling with Photo Voltaic (PV) Panels

Sunlight

Photovoltaic Panels

Electricity

Comfortably
Cool Building Vapor Compression Chiller


Coefficient of Performance (COP)
Heat Out = Cooling ÷ Electricity Input
≈ 3.0 for air-cooled
≈ 5.0 for water-cooled
Condensation

Electricity
High Pressure Side
Input
(Hot Refrigerant)
Expansion

Low Pressure Side
Compression (Cold Refrigerant)

Evaporation

Heat In =
Cooling

Why are we using a water-cooled chiller system in the PV panel
analysis?
The answer is the same as why the first step in putting PV panels on
a home is to make the appliances more efficient
 The cost of the PV system is much more expensive than the cost of
upgrading appliances (in a home) or installing a very efficient cooling
system (in a building) so it makes economic sense to minimize the PV
panel quantity

Solar Cooling with Solar Thermal Panels

Evacuated Tubes

Sunlight

Solar Thermal Panels
Hot Fluid

Comfortably
Cool Building Absorption Chiller


 Single Effect Absorption
Cycle
Uses Ammonia or Lithium
Bromide as a sorbent
 Ammonia is an atmospheric
pressure cycle, LiBr is under a
vacuum
 There are very common in
applications where there is
waste (free) heat like in
hospitals and power plants


 Double Effect
Absorption Cycle
Uses Ammonia
or Lithium
Bromide as a
sorbent
 There are other
ways to achieve
double effect,
drawing shown is
only one
application


= Cooling ÷ Driving Heat Input
≈ 0.7 for single effect (180°F)
≈ 1.3 for double effect (350°F)

Evacuated Tubes

Comparing Rooftop Solar Cooling Options

Solar Cooling Efficiency (SCE) = Collection Efficiency * Cooling Efficiency
Collection Efficiency = Available solar energy converted to electricity (PV) or heat (Thermal)
Cooling Efficiency = Coefficient of Performance (COP) of the refrigeration process

SCEPV = 15% Collection Efficiency * 5.0VC = 75%

Removed from further
SCEST = 50% Collection Efficiency * 0.7SE = 35%
consideration
Evacuated Tubes

SCECST = 65% Collection Efficiency * 1.3DE = 85%
Fresnel Concentrator

Roof Area (not including service access clearances)
Refers to actual solar panel area
Roof footprint may be different, panels assumed to be on 25° angle

AreaPV = 66 square feet per ton
(COP = 5, 170 Watts AC, GE panel)

AreaCST = 62 square feet per ton
(COP = 1.3, Chromasun panel)


Note for a cooling load of 400-square feet per ton that solar can
cool at least two stories with ample roof area left over

September 2009 ASHRAE article
compared several methods of
harnessing solar power and
concluded that solar-thermal beat
photo-voltaic in Abu Dhabi

Solar Cooling Cost
Refers to new installations’ first-costs
Maintenance cost differences between PV and CST are insignificant
Balance-of-Plant cost differences between PV and CST are insignificant

CostPV = $5,000 per ton
(COP = 5, $7/Watt)

CostCST = $5,500 per ton
(COP = 1.3, $2,400/Chromasun panel)


Incentives and Rebates
Federal Investment Tax Credit (ITC) gives back 30% of the total system cost for a solar installation
Many local utilities have programs in place to absorb even more money (about 25%) of costs

Energy Payback Time (EPBT) = Energy to Manufacture ÷ Annual Output
Energy to Manufacture = Requirement to manufacture solar collector
Annual Output = Useful energy output from the collector over a one year period

EPBTPV = 3 years to 7 years
(7 years figure from: Blakers, Weber, The Energy
Intensity of Photovoltaic Systems, October, 2000)

EPBTCST = 0.7 years

Recyclability at the end of panel life

“The most widely used solar PV panels...have the
potential to create a huge new wave of electronic
waste (e-waste) at the end of their useful lives...new
solar PV technologies are increasing cell efficiency and
lowering costs, but many of these use extremely toxic
materials or materials with unknown health and
environmental risks.”
- “Toward a Just and Sustainable Solar Energy Industry”, Silicon Valley Toxics Coalition (1/14/09)

CST panels have aluminum frames, steel pipe
(receiver), and tempered glass covers. The unit is
fully recyclable except for the sealing compound at
the glass/metal interface, a small control board, and
black receiver paint.

Ulterior Benefits of a PV system
PV is more common than CST and has industry inertia
behind it (easier to permit and get competitive rates)

Objectives Summary
Describe two practical methods for solar cooling
Photo-voltaic (PV)
Concentrating Solar Thermal (CST)

Give air conditioning engineers the confidence to
offer customers a mechanical approach to solar power
CST slightly beats PV in efficiency
CST wins over PV in cost and moves solar power monies to
the mechanical scope
CST obviates PV when considering environmental impact
PV is much more popular than CST in the current market

Help engineers understanding when and how to
apply solar cooling systems

Harnessing the Sun as a Green Technology for a Building
Is the sun’s energy best-suited for offsetting base loads or transient loads?
What kind of building load correlates well with sunlight?

Can the transient sun satisfy the entire cooling load of a building?
A better question is can the environment satisfy the entire cooling load?

Can the Environment Satisfy All Cooling Loads? (Yes!)
When outside air is cold enough to satisfy cooling load – use airside economizers
When air is not cold enough but sun is out – satisfy load with solar cooling
Elevating supply air setpoint achieves sustainable cooling with some free heating

The Solar Multiple
Should the Environment Satisfy All Cooling Loads?
Recall Utilization: Economics of a green technology are optimized
when the most expensive components operate 100% of the time
Solar panels (CST or PV) are by far the most expensive components
of a solar cooling system
Solar cooling should possibly be sized for less than the total load to
maximize utilization (Solar Multiple < 1)

Solar Multiple =
Maximum Cooling Output of Solar Panel Array
Design Cooling Load of Building

Correlation Sizing Strategy: Size the solar cooling system to satisfy
only the solar-dependent components of the building’s cooling load
This sizing strategy maximizes correlation and utilization

CST or PV Solar Cooling: Utilization and the Solar Multiple
If Cooling SAT > 62°F it favors full-load free-cooling (Solar Multiple ≈ 1)
Solar Multiple < 1 in all other systems to optimize economics

PV Solar Cooling: Storage and the Solar Multiple
Benefits of a PV system with Solar Multiple > 1
“The grid” is a readily available and free storage system
Systems can be sized to make annual energy bills zero out (100% utilization)
A potentially less-expensive approach might be a hybrid CST and PV system

CST Solar Cooling: Storage and the Solar Multiple
Benefits of over-sizing a CST system (Solar Multiple > 1)
Building or campus loops are readily available and free storage systems
When CST output > building cooling load, the extra is still used “in-house”

Solar Collectors - Flat Plates

Sunlight
Evacuated Tubes

Hot Fluid

Flat Plate Collectors – Pros and Cons

Sunlight Evacuated Tubes

Hot Fluid

Advantages Disadvantages
Comparatively very inexpensive method for Heat of the fluid generated is not high grade
harnessing solar resource (only domestic hot water or pool heating)
Very common in the industry, competitive Large receiver surface area means high
products and installers are easy to find losses on cold, sunny days
Low energy payback time and almost fully Evacuated tubes usually lose their charge
recyclable after about 7-years
Qualify more easily for solar-thermal No way to turn panel off – controls
rebates necessary to prevent overheating

Photo-Voltaic Collectors – Pros and Cons

Sunlight Electricity
Photovoltaic Panels

Last longer than flat plates (20-25 years) Expensive method for harnessing solar
resource compared to flat plates
Very common in the industry, competitive Very high energy payback time, at end of
products and installers are easy to find life they become electronic waste
Qualify for electrical rebates, can take Comparatively very low efficiencies versus
advantage of the coefficient of performance flat plates and concentrator panels
The grid is free storage Do not qualify for thermal rebates (unless
heat recovery is accepted)

Concentrating Solar Thermal Collectors

Sunlight

Solar Thermal
Concentrating Panels
Hot Fluid

Concentrating Collector Types –
Parabolic Troughs and Compound
Parabolic Concentrators

Parabolic Troughs Compound Parabolic Concentrators (CPC)
Tracking necessary No tracking necessary!

Concentrating Collector Types –
Linear Fresnel Concentrators
Advantages over other
Concentrators
Lowest heat loss of all concentrators
Flat Panel shape for easy roof-mounting
(troughs are ground-mounted)
Easily stowed by moving mirrors (dissipaters
not required as opposed to CPC)

Fresnel Mirror Arrays

Concentrating Solar Thermal – Pros and Cons

Sunlight
Solar Thermal
Concentrating Panels Hot Fluid
Fresnel and Troughs: Last as long as PV panels Expensive method for harnessing solar
(20-25 years), internally controlled to “lose resource compared to flat plates
the sun” instead of overheating (CPC not so)
High grade heat (solar cooling and hydronic Not common in the industry, competitive
heating are easily viable) products and installers are hard to find
Qualify for thermal rebates, take advantage of Require storage tank for uncorrelated loads
heat pump coefficients of performance
Highest efficiency for harnessing solar energy

Flat Plate Collector Applications
= 1.0 for heating

Sunlight Solar
Thermal
Evacuated Tubes
Panels
Swimming
Pool Hot Fluid ≈ 130°F
Heating

Domestic Hot
Water Heating

Solar Thermal Temperatures
Solar Collector Performance Curves
100%
Ambient Temperature 20C
Solar Radiation 800 W/m2
90%

80%

70%
Collector Efficiency [%]

60%

Chromasun HT MCT
50%
Evacuated Tube
Flat Plate
130°F Line

40%
CPC

30%

20%

10%

0%
0 20 40 60 80 100 120 140 160 180 200
Temperature Difference (Tm-Ta) [C]

Solar Heating with Photo Voltaic (PV) Panels
Coefficient of Performance
= Cooling ÷ Electricity Input
≈ 2 for air-source heating

Sunlight

Photovoltaic Panels

Electricity

Comfortably
Warm Building Air-Cooled
Heat Pump

= Cooling ÷ Electricity Input
≈ 3.0 for air-cooled
≈ 5.0 for water-cooled
Sunlight

Photovoltaic Panels

Electricity
Water-Cooled
Chiller

Comfortably
Cool Building OR Air-Cooled
Heat Pump

Solar Heat Recovery with Photo Voltaic (PV) Panels
≈ 3.0 for cooling
≈ 2.2 for heating
Every Watt of sunlight becomes 5.2
Watts of useful thermal energy!

Sunlight

Photovoltaic
Cool Panels
Building
Electricity

Domestic Heat
Hot Recovery
Water Chiller

Solar Heating with Rooftop Concentrators
Solar Coefficient of Performance (COP)
Thermal Direct Heating = 1.0
Panels ≈ 1.6 Absorption Heat Pump

Sunlight

Hot Fluid ≈ 365°F
Domestic
Water
Heating
OR
Hydronic
Absorption Water
Heat Pump Heating

Solar Cooling with Rooftop Concentrators
≈ 1.3 Double Effect Absorption

Sunlight

Hot Fluid

Comfortably
Cool Building Absorption Chiller

Solar Heat Recovery with Rooftop Concentrators

Solar Coefficient of Performance (COP)
Thermal Every Watt of sunlight becomes 2.2
Panels Watts of useful thermal energy!

Sunlight

Cool COP ≈ 0.6
Hot Fluid
Building

Domestic COP ≈ 1.6
Absorption
Hot Heat Pump
Water

Conclusions – What to Remember
Applying a green technology where consumption and availability of
the green commodity are closely correlated has superior economics
Sunlight and solar-dependent heat gains are closely correlated
Applying a green technology where the most expensive system
component(s) are utilized as much as possible has superior economics
Solar cooling panel utilization is always high when sized for the solar
component of the cooling load (Solar Multiple < 1)
Coupling solar cooling and airside economizers gives the potential
for free cooling 24/7 in buildings with elevated supply air set points
(laboratories, data centers, classrooms, DOAS, UFAD)
Solar cooling adds substantial scope to the mechanical portion of a
job thereby increasing profits for air conditioning companies
Questions?

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