Residential Energy Use

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Residential Energy Use
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The industrial sector includes facilities and
equipment used for manufacturing, agriculture,
mining, and construction. The transportation
sector comprises vehicles that transport people
or goods, such as cars, trucks, buses,
motorcycles, trains, subways, aircraft, boats,
barges, and even hot air balloons. The
residential sector consists of homes and
apartments. The commercial sector includes
buildings such as offices, malls, stores,
schools, hospitals, hotels, warehouses,
restaurants, places of worship, and more.
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How Water Heaters

• Heated water is used in bathing and cleaning
  periodically throughout the day.
• Energy is required to heat the water.
• If not used immediately, energy is also required
  to keep the water hot!

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Types of Water Heaters Tank

• Water is
• heated and stored
• delivered to user when needed
• replaced in the tank by municipal connection
• Reheated as needed
• Sizes
• 60 90 gal. whole home
• 3 10 gal. point of use
• Energy Use
• Electric
• Natural gas
• Solar
• Heat pump

Homedepot.com
Heaterdetails.info
unitedinspection.com
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Types of Hot Water Heaters Tankless
Heaterinsigt.info

• Water is
• run through the heater when needed
• brought up to desired temperature
• delivered to user
• not stored
• Energy used
• Natural gas
• Electric

A-archer.net
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How to Choose?

• Not an unimportant decision
• 22 of US energy use is residential
• 20 of that is heating water

• Things to consider
• Size
• Fuel type
• Overall cost
• First hour rating

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Tankless water heat ratings

• Primary consideration want hot water at a
  desired flow rate
• Appliances are rated by the maximum temperature
  rise possible at a given flow rate

• Amperage (amps)  29 A
• Element Wattage (watts)  7KW
• Flow Rate _at_ 35F Rise (gallons/min)  2.2 gal
  (US)/min
• Flow Rate _at_ 45F Rise (gallons/min)  1.5 gal
  (US)/min
• Flow Rate _at_ 77F Rise (gallons/min)  0.5 gal
  (US)/min
• Fuel Type  Electric
• Maximum temperature (F)  125
• Voltage (volts)  240 V

Rheem Ecosense 7kW
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Sizing a tankless water heater

• Assume
• you live alone and just want a hot shower.
• a low flow 1.5 GPM showerhead (Federal Regulation
  from 1992 is 2.2 GPM max)
• 50 F inlet water
• You want a 95F shower.
• Is this possible with the 7kW Rheem?

• To size one we must estimate the peak usage
  flowrate, dV/dt, (usage) in volume/time
• Then, we must determine if the system can heat
  that amount of water per unit time from the
  starting to the final temperature.
• Finally total daily usage tells us how much
  energy is used

Typical Flow Rates in Gallons per Minute (gpm) Typical Flow Rates in Gallons per Minute (gpm) Typical Flow Rates in Gallons per Minute (gpm) Typical Flow Rates in Gallons per Minute (gpm) Typical Flow Rates in Gallons per Minute (gpm)
Fixture Type Bathtub Shower Kitchen Sink Dish-washer
Flow Rates 2.0 4.0 1.5 3.0 1.0 1.5 1.0 3.0
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How much energy is required?

• Assume we take a shower at 1.5 gpm
• The outlet temperature will be 95F (50F45F)
• How much power (dQ/dt) is required to achieve
  this change in water temperature?

Hmmm!! Someone is not telling the truth! Where
are their ethics?!
If your luxury shower takes 10 minutes how much
energy is used? Q10,100 J/s (600 sec) 6.01
MJ (or 2.5 Big macs)
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What about your dorm room or apt.?

• Peak usage
• 1 shower at a time
• 1 bathroom sink
• 1 Kitchen sink (or not?)
• The power required at peak would be the sum
  required from all sources used at the same time.
• EX Two 1.5 GPM showers at 95F (from 50F) would
  require a unit capable of 20.2kW by our
  calculations.

• Total Daily Usage
• of people bathing
• Dishwasher, sink, clothes washing
• The daily energy required is found by integrating
  (summing) power usages throughout the day
• If one bather took 5 and the other 10 minute
  showers, you would use 6MJ 3MJ9MJ of energy.

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What About your parents home?

• Your parents home
• Peak usage
• 2 showers concurrently in the morning (possibly
  multiple sets of two)
• 2 sinks in the morning (possibly multiple sets
  of two)

• Total Daily Usage
• of people bathing
• Dishwasher, sink, clothes washing

Total daily use tells the amount of energy used
per day.
Peak power tells the size of the unit required.
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Worksheet for Estimating Daily Water usage Say for a family of 4 Worksheet for Estimating Daily Water usage Say for a family of 4 Worksheet for Estimating Daily Water usage Say for a family of 4 Worksheet for Estimating Daily Water usage Say for a family of 4 Worksheet for Estimating Daily Water usage Say for a family of 4 Worksheet for Estimating Daily Water usage Say for a family of 4
Use Average gallons of hot water per usage Times used during day   Gallons used per day
Shower 12 3 36
Bath 9 1 9
Shaving 2 1 2
Hands face washing 4 4 16
Hair shampoo 4 2 8
Hand dishwashing 4 0 0
Automatic dishwasher 14 1 14
Food preparation 5 3 15
Wringer clothes washer 26 0  0
Automatic clothes washer 32  1  32
      Total Daily Demand  100 gallons
How much energy would be required to heat all of
this water by 45F?
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Sizing a tank (storage) water heater
40 gal GE hot water heater

• Fuel Type  Gas
• Gas Type  Natural Gas
• Total BTU  40000 Btu (per hour)
• Maximum temperature (F)  160
• Minimum Temperature  60 F
• Recovery Rate at 90F Rise (GPM? GPH)  40.4
• Tank Capacity (gallons)  40 gal (US)
• First Hour Rating (GPH)68.0
• Tank Diameter (in.)  20.25
• Tank Height (in.)  62
• Beware of faulty information

• primary consideration the tank should not run
  out of hot water examine the first our demand
• Important specifications include
• size (gal.)
• Energy used (often in BTUs)
• Recovery rate time to reheat water and First
  Hour rating
• energy factor (EF) relative measure of hot water
  produced per unit energy
• R value of tank insulation (sometimes not given)

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Worksheet for Estimating Peak Hour Demand/First Hour Rating Worksheet for Estimating Peak Hour Demand/First Hour Rating Worksheet for Estimating Peak Hour Demand/First Hour Rating Worksheet for Estimating Peak Hour Demand/First Hour Rating Worksheet for Estimating Peak Hour Demand/First Hour Rating Worksheet for Estimating Peak Hour Demand/First Hour Rating
Use Average gallons of hot water per usage Times used during 1 hour   Gallons used in 1 hour
Shower 12  2  24
Bath 9    
Shaving 2  1  2
Hands face washing 4  4  16
Hair shampoo 4    0
Hand dishwashing 4    
Automatic dishwasher 14    
Food preparation 5  1  5
Wringer clothes washer 26    
Automatic clothes washer 32    
      Total Peak Hour Demand  47 gallons!! We are close. Possibly undersized.
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How much energy is required to heat the water in
the tank?

• Assume the water enters the tank at 50 F.
• The energy required to heat 47 gallons to make it
  available?
• But if we do not use it immediately, then it will
  cool off and we will have to heat it again!!

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How much heat is lost by the water in the tank?

• Convection will occur at outer surface of the
  tank.
• Difference between air temperature and surface
  temperature drives convection.
• We know Tsurface gt Tair so we draw convection
  leaving tank.
• Since heat is being lost, dQH/dt, we must reheat
  dQgen/dt, to maintain temperature. Want dT/dt0
• We set TH20 and we can measure Tair. This is all
  we need to find heat flux.

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Combined convection, conduction problem

• Examine control volume (dotted line) and
  boundaries. Heat flow is constant.
• Examine Water/insulation surface
• Examine conduction thru insulation
• Examine insulation/air interface

insulation
Water
Th20
Tair
Tinner
Touter
d
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ConductionConvection Through multiple thermal
resistances (R-values)

• If we sum the boxed equations
• In general the thermal resistance for conduction
  is
• Thermal resistance for convection is
• And for any thermal pathway in 1D, the heat flux
  from the interior to the exterior is (where a
  negative sign means a reversal of direction)

Note that in our problem, this is the power
required to maintain the temperature of a full
tank. This is why you insulate your water
heater!!! If water is used, then additional heat
is required to increase the replacement waters
temperature from ambient, to our set point.
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Water Heater Heat Loss

• If the water heater is well insulated, k0.05
  W/m-C how much power on average is required to
  maintain the temperature in the tank for 24 hours
  assuming worst case natural convections?
• Assume the tank has
• Ainner AouterA2prheight2pr2
• Where r10.125 inches, height62 inches
• How much energy is used in that 24 hours?

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Costs?

• Energy use cost
• Electricity 0.18 kW-hr
• Gas
• Installation
• Tankless
• Gas 2200
• Electric 240 V (2200) or 110V (1350)
• Tank
• Gas800
• Electric 110V (150) (or 240V (800)

• Given our average daily usage, we can determine
  the recurring costs.
• Given the installation costs, we can determine
  the initial costs.
• Given a prevailing interest rate, we can
  determine present and future values.