Solar Hot Water

Which is better?

PV + Heat Pump or Thermal

http://www.halslater.com/Solar-Hot-Water.htm

Solar thermal is

temperamental, really.

• Water weight limits design

• Expands when it freezes

• Steam and scale damage pipes when it boils

• Solar thermal systems are efficient but rarely use all of it

• Some systems work fine for decades, with regular inspections

• When solar thermal system fails it tries to destroying itself

• Use STHW when regular maintenance can be assured

The wisdom has been…

• Efficiency outweighs convenience of electric.

• Since thermal collects more energy, PV powered

conventional electric will never compete

• Then 2.5 EF heat pump water heaters arrived

• Then the cost of PV fell by 75%

The New Reality

• Lower solar electric (PV) costs; $3.00-$5.00/Watt

• DOE SunShot 2020 targets: $1.50 res; $1.00 util

• A new model: air source heat pump water heating

(AWHP) with solar electric offset

• Heat pump is mature technology

• AWHP has fewer drawbacks than solar thermal

• Has a smaller price tag for residential applications

Has already begun

PV+HP Assumptions

• Using a heat pump water heater with…

• Energy Factor of (EF) of 2.5 or more

• An 1,800 kWh per year Energy Star rating

• Adding a grid-tied NEM system with…

• 1.0 kW to 1.3 kW of PV (50-90 sf)

• Where PV produces at least 1400 kWh/kWp/year

PV Advantages

Lower upfront cost

• Low-cost open systems proven unsuitable

• Cost of 80% solar thermal

• closed loop - glycol or drainback

• two-tank - or storage plus tankless

• 40 - 64 square feet of flat-plate collector

• Total cost $7,000 to $10,000 before incentives.

• Cost of 100% PV offset heat pump water heater

• $1,500 to $2,500 for the heat pump with labor

• $3,500 to $6,000 for grid-tied PV

• Total cost $5,000 to $8,500 before incentives.

Easier to install

Photovoltaic

• Replace a water heater with

another single tank

• Add three to five modules

to PV system.

Solar Thermal

• Replace a single tank with

two tanks

• Pipe heat transfer fluid to

rooftop panels

• Pressure test and charge

after installation.

or

Which has fewer opportunities for installer error?

Uses less space

• Two tanks are required for thermal

• One for the backup, and one for the solar

• Competing with backup limits solar fraction to ~60%

• Can use tankless heater if it modulates enough

• Adds to equipment and installation cost

• May need larger gas line

• Cold water sandwich effect

Needs no maintenance

• The Achilles Heel of solar thermal is…

• If the system stops working, it wants to self-destruct

• Without flow, the panels can…

• Freeze, warp, boil, scale, coat, burst and/or clog

• These should be inspected yearly

• Panels

• Electronic controller

• Circulator pump

• Piping for scale or corrosion, especially for drainback

• Older buildings that may settle and trap fluid

• Annual inspections could cost half of the gas savings

Cannot freeze

• Solar panel can freeze at 42ºF

• Freeze protection is required for all mainland US

• Except for drainback, freeze protection is "active”

• Need device to operate only at low temperature.

• Rarely required to function

• Failures are common and catastrophic

• Thousands of dollars in damage to collectors

• Systems turned off then removed v. repaired

Cannot overheat

• Frequently overlooked in product and system design

• Twice as much solar energy in July as in January

• Enough heat in January will overheat in July

• Panels stagnate at 400ºF inside

• Damages and accelerates deterioration

• No data on how radiator much to use

No scale build up

• Scale is the enemy of water heaters of all types

• Heat makes dissolved solids precipitate from water

• They bind to hot surfaces

• Scale can clog the heat exchanger

• Lower temperatures in heat pump reduce scale

100% solar fraction

• No 100% reliable thermal system has 100% fraction

• Best SRCC OG300 systems have 90% solar fraction

• NEM allows storage of power for up to a year

• Comparison is 80% for thermal but 100% for PV

Demand management

• AWHP adds load to the grid when replacing gas

• PV adds power to the grid during peak demand hours

• Most domestic hot water is used off-peak

but

• The utility could use this to store excess electricity…

• Add ability to overheat water through the smart meter

• Use with a mixing valve to prevent scalding

• Remotely manage operation of the compressor or element

• Store twice as much energy as used with AWHP

Net gas efficiency

Electric Tank

Gas Tank AWHP

Gas Tankless

Rated EF 0.92 0.67 2.5 0.92

Elec:Gas EF 0.33 1.0 0.33 1.0

Net Gas EF 0.30 0.67 0.83 0.92

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4

No CO2 emissions

• Gas combustion backup adds CO2

• IPCC says it is the #1 risk facing civilization today

• PV + HP produces no GHGs while operating

• Lower GHGs in producing less equipment

Disadvantages

Warm air required

• Efficiency depends upon heat source for air in space

• In temperate climates, yields higher EF

• If space is heated, energy comes from heat source

• If cold, use of backup element lowers net efficiency

• APWH will cool and dehumidify the space

Newer on the market

• Household AWHP only 20 years of development

• First were add-on units

• 2004 ORNL optimized single tank design

• Energy Star rated water heaters:

• 500 solar thermal models

• 600 tankless ("instant”)

• Only 23 APWH models.

• Proven core technology – vapor compression

Noise level

• Heat pumps use compressor and fan for air flow

• As much noise as a small, modern refrigerator

• If the unit is inside, the sound may be a problem

• Fan air makes more noise than compressor

Conclusion

• Solar thermal technology is an improvement over fossil

• Has legitimate applications with regular maintenance

• Household-level solar hot water has many drawbacks

• Solar photovoltaic is highly-effective, low maintenance

• APWH is the optimal selection for many households,

depending on climate and configuration

• W2W heat pump water heaters may become available

Question & Answer

Best LinkedIn response:

• “This is a discussion that will have as many opinions as there are readers. One reason that thermal gets a bad rap is our past history in the 80s where open loop systems with freeze valves or sunspool valves failed and systems froze. Then we have had instances of glycolstagnation issues turning acidic and into a jello like substance

which plugged the heat exchangers. Here in Florida, we are seeing

issues with water quality for various reasons; scaling from calcium

carbonate and copper corrosion from salt water intrusion to fresh water sources. Normally, calcium and chlorine are not huge issues in residential plumbing but when you pipe the water in an open loop system

and run it through a collector that might stagnate to 250F, you

accelerate the reaction rates dramatically.

• Yet, we now have strategies that can handle these, ie closed loop drainback systems.”• John L Alger, P.E., Manager, Engineering and Commercial Projects, AET Solar, LLC

Thermal Advantages

• “Plus, consider that you need much less roof area for collectors. My home has a 4.86kW PV array which requires 27 panels. My thermal array is 2qty 4x8 panels [64 sf] and generates the same equivalent. Why would I use such a precious and sophisticated product like electricity, just to warm up water? The use of both technologies is really the best.”

The 1-1.3kW array needed will be 50-70 sf. I do not see a real difference in the sophistication of

the energies or the relevance. AWHP uses mostly air.

“With regards to heat

pump water heaters…

New data suggests that there are some significant considerations including

but not limited to:”

I will respond.

1. Weight of the unit

• “In some cases the flooring needs to be reinforced”

Why? The main issue is still the weight of 50 gallons

of water, the compressor adds under about 100 lbs.

2. Maintenance

• “It is an air conditioner with a filter that needs to be replaced.”

That is not mentioned in any of the manuals I found but is a very minor action, if true.

3. Access

• “It can not be placed in a position next to a

wall where the flow of air is impeded.”

That depends on the air flow direction through

the heat pump, the unit can be rotated.

4. Design life

• “While the HVAC side will last quite a while, 15 years perhaps like a heat pump, the actual durability is only as good as the life of the steel tank, Then, you have an expensive replacement in 6 to 10 years vs solar with a SS tank which does not rust or a DX drainback system which uses a simple steel glass lined tank that is inexpensive.”

The lower temperature of the wraparound condenser in the tank should eliminate the typical scaling found in water heaters but there are stainless tank HPWHs available, as well.

5. You need to add a

condensate line

• “Like an A/C system. That may not always be convenient to route depending on where the tank is located in the home.”

In most cases, the condensate line can be shared with the required T&P line

6. It works best in hot

climates• “In northern areas, with cold water temperatures,

the unit works mostly like a standard electric water

heater since there is not enough latent heat energy in

the surrounding air to transfer to water in the tank.

this is the same reason that heat pumps are not

commonly used in northern climates for space

heating, the COP is too low to deliver any heat.”

True, depending upon what is meant by “northern areas”.

Many labs and manufacturers that test and build AWHPs

are in the northern states.

7. Who to call for service?

• “Plumbers cannot service an HVAC system.”

Why not call an HVAC technician? Plumbers

rarely perform any service on a water heater

other than replacement, anyway.

“There has been much talk

• … lately about a PV powered electric water heater but again, why take a technology that can generate a product which is so versatile (electricity) and use it to warm up water?”

You answer your own question, electricity is “versatile” so, if you don’t need warm water, you can still use the electricity.