An Ontario geothermal installer tells me that geothermal systems won't work for a house powered by PV solar panels if it's off the grid, as the panels can't provide the power needed for the startup surges/spikes of the geothermal system. Can anyone verify or debunk this? Thanks.

It isn't the panels, it's the inverter. And it all depends on the size of the geo system and inverters used. I know I can't start our geothermal system if we are off grid, but if we are grid tied I can start the compressor and then drop from the grid and keep running. We have a 4 ton geothermal system and a XW 6048, 6000 watt continuous. It is suppose to supply 18kw surge, but it can't start our system. I have often wondered about staggering the motors start time, maybe trying to just start the main motor then turn on all the circulation pumps a few seconds after that.

I can start our 2 HP pool pump, I am not sure what the HP rating is on the main geothermal pump. I think the problem is the compression starts hard.

Figure about 1 HP per ton. Staggering starts might work, but the compressor is so much larger than the blower and circ pumps that I doubt it would make much difference.

Starting current tends to run about 5.5 times nameplate running load amps, and most systems' actual running amps are well below nameplate, but lower running load amps doesn't reduce starting current.

The nameplate should include a (very large) figure for LRA - Locked Rotor Amps. That is equivalent to starting current for a split second.

My 25 KW continuous, 43 KVA surge backup generator audibly bogs down a bit and lights flicker when my 3 ton, 83 LRA geo starts. My running amps are about 6 cooling, 7 heating.

Engineer, do you understand why heat pumps draw more amps when heating vs cooling? Mine is about 21 for heating and 15 for cooling.

I've noticed the same thing for my GSHP units - a higher real-time KW consumption for heating vs cooling.  Noticed it this month because we were still using some cooling at the beginning of this month, now we're using (just a little) heating.

For example, my steady state power consumption for my 3 ton unit in 1st stage is 1.6 KW in cooling mode - it rises to a steady state consumption of 2.0 KW in heating mode.

At the same time, my compressor R410A discharge rises from about 120°F in cooling mode to about 155° in heating mode.

Don't know enough about refrigeration to understand why.  But I thought I'd report that I see a similary phenomena, and I look forward to learning from others here who understand this stuff.

Best regards,

Bill

To understand the increased power in heating mode consider these things:

1) In heating mode, system's sides swap places - condenser heats air rather than water; evaporator cools water instead of air.

2) In cooling mode we strive for lower leaving water temperatures (LWT). In many systems, the LWT during cooling is below what we'd consider a comfortable and desireable Leaving Air Temp for heating. Why does that matter? It leads to the fact that in many, likely most, systems, the condenser during heating (Air coil) runs at higher temperature than the condenser during cooling (near LWT). Most systems' leaving air temp is 90-100 F, and ideally toward the upper end of that range.

3) Higher temperature condensing means higher refrigerant pressure, which means higher compressor power. The observed increase in compressor discharge temperature supports these observations. Discharge temp is the sum of saturation temperature plus superheat both carried in from evaporator and added by compression.

4) A related aspect is that water heat exchangers operate more efficiently (both fluids' temperatures get closer together) than air heat exchangers. Heat flows more easily into water (cooling mode) than it does into air (heating mode)

Hope this helps.

BTW - that 155 compressor discharge is most excellent for desuperheat / hot water generation, even though it somewhat detracts from system's space heating capacity.

Posted By engineer on 11/21/2008 8:23 AM
To understand the increased power in heating mode consider these things:

...

3) Higher temperature condensing means higher refrigerant pressure, which means higher compressor power. The observed increase in compressor discharge temperature supports these observations. Discharge temp is the sum of saturation temperature plus superheat both carried in from evaporator and added by compression.

...

BTW - that 155 compressor discharge is most excellent for desuperheat / hot water generation, even though it somewhat detracts from system's space heating capacity.

This is very helpful - much appreciated.

I can understand point 3 above.  My compressor is operating with it's R410A refrigerant at a higher pressure in heating than in cooling mode.  Thus, it makes sense that current draw has to increase - the compressor is working harder.

With respect to the DSH:  Last year I turned it off during cooling mode after figuring out how poorly it performed (thoroughly discussed in other threads - not looking to discuss it here).  I left it off during the heating season, reasoning, without data though, that it probably similarly poorly performed.

During this year's cooling mode, after further study (again, well discussed elsewhere) I turned the DSH back on.  Being curious to see how it actually performs in heating mode, I've left it on.
 
A key new learning already for me is the substantial increase in compressor discharge temp.  I didn't know, even with an R410A system, the amount of 'headroom' available to heat domestic hot water (DHW) would increase so much (for me, from about 120° to about 155° - 3 ton, 1st stage unit).

For this Winter I have a monitor set up to track the total energy required to provide DHW, and the portion of the total supplied by the DSH.

For this month so far the portion is only 2%, but that's not surprising.  We're in this 'in-between period' going from Summer to Winter, where we run little HVAC, cooling or heating, at the moment.  The month of Jan., being our most intense heating month, should illustrate best possible DSH performance.

Many thanks again for the instructional comments.

Best regards,

Bill