WtW Efficiency gain of lower temp. Load Tank

[buffalobillpatrick]

"You'll want to add a valve in the solar direct connection to prevent stray flow"

Would this be a flow check valve added above & left of P5 in the cold return line from P6?

"you'll do better putting one in the main loop so it can also be used to help overcome restriction for direct solar mode."

Both pumps (P5 & P6) that pull heat from solar tank HX will be Taco 008 high head.

When HP is heating DHW only P7A would be creating flow in main loop, it could be too low for HX to operate propperly? The HX has very low head loss, GEA has a great online HX simulator that I used to select.
I will have to watch for this.

I think "balance point" means where HP would stabilize operation?

"the balance point for 120-130F condensing would be way below 90F evaporating"
I don't know enought to know what impact this would have? Hotter water out of condenser?

"And how would you handle the corner case where both zones are calling but the source temperature is between the two balance points?"

Most boiler systems give priority to DHW, lots of control system details need to be worked out.
BTW, these solar temps. are a first guess & all will be adjustable as required.

Goals from where I got this main loop idea:

Simple to understand & maintain.

Heat sources from low to high temp & load from high to low temp.

1 or 2 sources providing heat at once & 1 or both loads drawing heat at same time.

ie: solar could be preheating input to boiler.

[jeff5may]

Originally Posted by NiHaoMike

Of which, exactly how are the solar panels/collectors coupled to the solar tank? If possible, it should be designed so that the heat be directly used (if needed) rather than stored.

That's what I'm sayin'. Since you live in the Colorado mountains, I'd definitely go with some sort of drainback system that feeds from and drains back into the solar heat store. This type of design avoids freezing and stagnation boiling issues that could both occur in your environment.

BBP,
Your understanding of the heat pump's purpose needs to be expanded for your system to operate at high efficiency. Let me open up the can of worms here.

The main issue concerning its operation is the ultra-wide open window of evaporator and condenser temperature. The compressor doesn't usually transfer the exact amount of heat printed on its nameplate. This capacity is determined under some ASHRAE test conditions in a lab. During normal operation, the actual amount will vary above and below this value. In your conditions, the amount of heat transferred will vary widely. Here's why.

The only thing the compressor does is move gas. The volume of gas it moves depends on suction pressure and compression ratio. The higher the suction pressure, the more mass it can compress. The higher the compression ratio, the harder it has to work to move this mass, and the less mass it will discharge on each stroke. Good mass flow is a key component to good heat transfer.

What this means is that at 40 degF supply water, the suction pressure will be much lower than at 80 degF supply water. As a result, the compressor will have much less suction pressure to draw from and lower heat transfer will follow. Using a TXV or EXV will amplify this correlation. This is a good thing: you don't want to flood nor starve your compressor. But this is your low side limit: mass flow follows evaporator temperature.

On the high side, mass flow works against condenser temperature. When the condenser is cool, the compressor operates at a much lower compression ratio due to the rapidly condensing refrigerant at low saturation pressure. It moves a lot of gas with less power (this is good). When the condenser is hot, the compressor has to work against an elevated saturation pressure. It has to work a lot harder to move not as much mass, so power draw increases and heat transfer suffers as well (this is bad). If the condenser pressure rises high enough, the compressor will overheat or piping will explode (very bad), since above its critical pressure, the refrigerant cannot exist as a liquid.

Also, since it is a water-to-water unit, you have three heat flow transitions to consider (one in each heat exchanger plus the low-to-high side transition). All three transitions must have a reasonable delta T for efficient heat flow. Simply slowing a water pump to increase delta T in a plate hx may actually murder heat transfer someplace else (compression ratio, suction pressure, etc). This is the balancing act: You need to define conditions that move heat well throughout the entire range.

With all of the pumps and flow paths in your system, it will take lots of work to optimize anything. Even with a supercomputer simulating operation, lots of trial and error is in your future. I wish you the best of luck and speed.

[buffalobillpatrick]

When I Google "Heat pump balance point" this is what I get:

The balance point for heat pumps is determined by your home’s heating load and your heat pump’s heating capacity. When your home’s heating load exceeds the heat pump’s heating capacity (in other words, when your heat pump can’t keep up with the cold weather outside), backup electric-resistant heating is activated, and your heating bill goes up and up. The outside temperature at which this occurs is your heat pump’s balance point.

[buffalobillpatrick]

jeff5may thanks, for some reason I can't click the "Thanks" icon as it don't appear right now.

Yes, the panels will heat the solar storage tank via a drainback open design promoted on the builditsolar.com site.

They will be ground mounted on a steel used well pipe rack behind house. Either vertical or close to it, as summer hail storms are a concern & with a snow cover they should perform well.

I will be looking to buy 2 more of the 4x10' panels, got the 1st set of 6 cheap off CL.

[NiHaoMike]

When it's a dual fuel setup like yours, "balance point" is the point where it transitions from the heat pump being more efficient to the boiler/furnace being more efficient.

By making it two loops with the option to link them (more or less what I designed), you can do what's optimum for both zones. Adding a desuperheater to the heat pump can make the DHW side more efficient.

[AC_Hacker]

Originally Posted by buffalobillpatrick

...I will be looking to buy 2 more of the 4x10' panels, got the 1st set of 6 cheap off CL.

BBP,

Have you done an estimate as to how much heat gain you will get from your solar collectors?

-AC

[buffalobillpatrick]

"You'll want to add a valve in the solar direct connection to prevent stray flow"

Would this be a flow check valve added above & left of P5 in the cold return line from P6?

[buffalobillpatrick]

A/C a guess for 240 ft2 = 30mbtu / yr

[NiHaoMike]

Originally Posted by buffalobillpatrick

"You'll want to add a valve in the solar direct connection to prevent stray flow"

Would this be a flow check valve added above & left of P5 in the cold return line from P6?

That's the two way valve in my design.

[AC_Hacker]

Originally Posted by buffalobillpatrick

A/C a guess for 240 ft2 = 30mbtu / yr

BBP,

The success of your project, in terms of non-fossil fuel gain, is entirely dependent on solar gain. I have been wondering, through this whole dialog saga, what kind of analysis has been done to quantify the rate of solar gain?

Is your plan to just keep adding solar collectors (and maybe water storage tanks) until you get it right?


So 240 ft2 would mean 6 collectors, each producing their share of 30mbtu, or each one producing 5mbtu/yr.

Do you mean that each one produces 5,000,000 btu/yr?

If so, then I think that would mean then, that each one produces 5,000,000/365 = 13699 BTU/day.

As I recall, the rule of thumb is that, in reality (adjusting for the ever-changing angle of incidence of the sun), you get 6 hours of useful solar energy per day, so that would mean that each panel will need to produce 13699/6 = 2283 btu per hour per panel or 57 BTU/ft2, 17 watts/ft2. (all assuming 100% clear days and perfect alignment of the collectors).

...and you have 1000 gallons of water in the tank, so that would mean 8,345 lbs. of water and since you have 240 ft2 of collector to heat your water, each square foot of panel would have to heat 8345/240 = 35 lbs of water.

So... each square foot of your solar collector array will ideally produce 57 BTU of heat energy, and that heat energy will be applied to 35 lbs of water (if all goes well).

Hmmm...

As I recall, you said that your house design would have a max rate of heat loss of 25,000 BTU/hr.

Assuming that this heat loss was acting over the course of a night, that would mean that in a night (12 hour) you would lose 25,000 x 12 = 300,000 BTUs total.

If your 8345 lb storage tank was at 180F as night fell (it could happen), then your 300,000 heat loss would deplete the tank's stored heat by 300,000/8345 = 36 degrees. So, in the morning, when you woke up, you'd see a temperature level of 144 in the tank... that would be pretty useful for washing dishes and more than enough for a shower.

The worst case would be if your tank was at 68F or less as night fell, then in the morning you'd be facing a tank that was at 32F.

It looks like you might be in the ball park, if your initial assumption of heat produced by your solar collectors is actually valid.


On the other hand, in the non-winter part of the year, your solar collectors will be capable of producing far more heat than you'll use in the winter months.

You just gonna let them sit there and bake?


-AC