Hi, Xin
I have not used the GSHE myself.
I understand the G-function is the variation of temperature with depth. The deeper it goes the cooler it becomes, because the deep ground temperature is at 18°C or lower in Winter.
If you are not using the slab program to calculate the ground temperature, you may increase the building surface ground temperature to 25°C or higher to increase the HE temperture.
If only the first three days were high and then drop off, this is the starting error due to the assumtion of the unknow history for digital calculation. EPlus assume 23°C for all the starting temperature variables. In Summer this is not too far from the real data. In Winter this is high and then reduces as more temperature data are available.
You may use a monthly cycle and choose a multiple simulation, say, 3 times using the same weather file input. The first cycle would be warm, the following Jan1 should be the real situation. The cause is again the low ground temperature in Winter.
The water flow rate is also the heat transfer rate. The tube temperature would be the ground temperature at low flow rate. On JAN1, when the flow rate is low, the tube temperature would be low. The G-function may include the effect of the outdoor surface ground temperature which is near 0°C.
The slab program may give you some value to change the G-function. The procedure may be in the auxiliary program manual of the engineering manual. The calculation may not be accurate, because the soil condition is not known. That is why I use the building ground temperature to move the whole system temperature. Your Winter ground monthly average may be set at 18°C. Increasing it to 25°C will be quite significant.
The WinterDdesignDay sizing simulation should give you a different value.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: xing.shi.2006@xxxxxxxxx
Date: Thu, 11 Aug 2011 04:55:31 +0000
Subject: [EnergyPlus_Support] Re: Ground source heat pump hot water supply T
__._,_.___
Primary EnergyPlus support is found at:
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The primary EnergyPlus web site is found at:
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The group web site is:
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I have not used the GSHE myself.
I understand the G-function is the variation of temperature with depth. The deeper it goes the cooler it becomes, because the deep ground temperature is at 18°C or lower in Winter.
If you are not using the slab program to calculate the ground temperature, you may increase the building surface ground temperature to 25°C or higher to increase the HE temperture.
If only the first three days were high and then drop off, this is the starting error due to the assumtion of the unknow history for digital calculation. EPlus assume 23°C for all the starting temperature variables. In Summer this is not too far from the real data. In Winter this is high and then reduces as more temperature data are available.
You may use a monthly cycle and choose a multiple simulation, say, 3 times using the same weather file input. The first cycle would be warm, the following Jan1 should be the real situation. The cause is again the low ground temperature in Winter.
The water flow rate is also the heat transfer rate. The tube temperature would be the ground temperature at low flow rate. On JAN1, when the flow rate is low, the tube temperature would be low. The G-function may include the effect of the outdoor surface ground temperature which is near 0°C.
The slab program may give you some value to change the G-function. The procedure may be in the auxiliary program manual of the engineering manual. The calculation may not be accurate, because the soil condition is not known. That is why I use the building ground temperature to move the whole system temperature. Your Winter ground monthly average may be set at 18°C. Increasing it to 25°C will be quite significant.
The WinterDdesignDay sizing simulation should give you a different value.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: xing.shi.2006@xxxxxxxxx
Date: Thu, 11 Aug 2011 04:55:31 +0000
Subject: [EnergyPlus_Support] Re: Ground source heat pump hot water supply T
Dr. Li: this problem really puzzles us. Let me explain the situation more clearly:
The HP heating serves a zone, about 5000 m2, in a not-so-cold city. The average dry bulb daily temperature in winter is about 3 degree C. The design is such that the hot water flow rate is adjusted and T remains at 45 degree C. We used Controller object in E+ to model it.
The U-shape vertical GSHX has about 200 bore holes and each is 58 m deep. The diameter is 140 mm and the design water velocity inside is 0.6 m/s. We know all these factors but not the G-function. Therefore, we copied the G-function from the E+ example file GSHPsimple. We know that it is a quite small zone and ours is big.
The E+ results show that the HP supply hot water stays at 45 degree C for several days (starting from Jan 1) and then drops down to below 20 degree C. We tried to solve this problem for quite a few days but failed.
Is it possible that we have to use the correct G-function to get the HX right? Anyway we can get the correct G-function?
Many thanks!
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@...> wrote:
>
>
> The set point number is used in a number of ways. In the air loop, the thermostat set point is used to turn on and off the supply. In VAV because the controlled variable is air volume and not temperature, the zone temperature may not follow the zone thermostat set point. The VAV set point is usually at the output of the fan of the air system, and is usually lower than the zone thermostat set point. The VAV damper controls the temperture in each zone. If the zone temperature is still too low at the minimum air flow damper setting, the reheat comes on. If you do not have the reheat, the zone temperature will follow the air system set point temperature. In the plant, the water is heated to the set point temperature and maintained at that temperature. At the coil, the outlet temperature of the coil will fall to the air temperature. The water at this temperature is returned to the boiler to be heated. If the boiler capacity is low, the set point many not be reached. Then the tank temperature will fall below the set point. This is the anwser to your question.======================In your case, you wanted the heat pump to give you a temperature of 45°C. You can watch the GS loop inlet and outlet temperature. For cooling the outlet temperature would increase. For heating the outlet temperature would decrease, because the inlet temperature is the temperature of the HP evaporator. The HP itself may stop when this temperature is at around 5°C, to prevent freezing. In a real system the auxiliary heater is turn on at this point. I do not know how your GSHE is defined yet. If you increase the ground temperature, the capacity should increase. Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: xing.shi.2006@...
> Date: Tue, 9 Aug 2011 23:44:52 +0000
> Subject: [EnergyPlus_Support] Re: Ground source heat pump hot water supply T
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> Thank you! A further question.
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> We are a little confused by the Setpoint function in EnergyPlus. let's say we model a Heat Pump to supply heating to a fairly large zone with significant heating load. For whatever reason, we model the ground heat exchanger in a wrong way so that it does not have the enough capacity to draw heat from the soil in winter. We use the setpoint object to set the heat water supply temperature at 45 degree C. In this case, would the temperature drop from 45 because the HX does not have enough capacity or it always stays at 45 because we use setpoint to force it at 45?
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> Thanks!
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> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@> wrote:
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> > I have not looked at your IDF yet. In general, GSHP do not have large heating capacity. If the Winter ground temperature is still at 18°C. it should work well. You may try increasing the capacity of the heat pump and the GSHE. Replace the HP with District heating and see what is the heating capacity required.
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> > Dr. Li
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> > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
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> > From: xing.shi.2006@
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> > Date: Tue, 9 Aug 2011 21:09:42 +0800
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> > Subject: [EnergyPlus_Support] Ground source heat pump hot water supply T
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> > We are modeling a 5 story building with two chillers and one ground source heat pump. Two chillers and the heat pump provide cooling in summer and the heat pump alone provides heating in winter.
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> > We tried to use the Controller:WaterCoil object to vary the heating coil supply water flow rate to change the heat provided to the zone. The heating supply water T is set to 45 degree C using SetpointManager:Scheduled and the node applied is HW Supply Outlet Node.
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> > We did not get any severe error. However, in winter we found that the HOT WATER LOOP:Plant Loop OutletNode Temperature [C](Hourly) varied from 45 degree C down to quite low. We were not sure about why this happend.
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> > Please help. Attached is the IDF file. Thanks! XS
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> >
>
The HP heating serves a zone, about 5000 m2, in a not-so-cold city. The average dry bulb daily temperature in winter is about 3 degree C. The design is such that the hot water flow rate is adjusted and T remains at 45 degree C. We used Controller object in E+ to model it.
The U-shape vertical GSHX has about 200 bore holes and each is 58 m deep. The diameter is 140 mm and the design water velocity inside is 0.6 m/s. We know all these factors but not the G-function. Therefore, we copied the G-function from the E+ example file GSHPsimple. We know that it is a quite small zone and ours is big.
The E+ results show that the HP supply hot water stays at 45 degree C for several days (starting from Jan 1) and then drops down to below 20 degree C. We tried to solve this problem for quite a few days but failed.
Is it possible that we have to use the correct G-function to get the HX right? Anyway we can get the correct G-function?
Many thanks!
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@...> wrote:
>
>
> The set point number is used in a number of ways. In the air loop, the thermostat set point is used to turn on and off the supply. In VAV because the controlled variable is air volume and not temperature, the zone temperature may not follow the zone thermostat set point. The VAV set point is usually at the output of the fan of the air system, and is usually lower than the zone thermostat set point. The VAV damper controls the temperture in each zone. If the zone temperature is still too low at the minimum air flow damper setting, the reheat comes on. If you do not have the reheat, the zone temperature will follow the air system set point temperature. In the plant, the water is heated to the set point temperature and maintained at that temperature. At the coil, the outlet temperature of the coil will fall to the air temperature. The water at this temperature is returned to the boiler to be heated. If the boiler capacity is low, the set point many not be reached. Then the tank temperature will fall below the set point. This is the anwser to your question.======================In your case, you wanted the heat pump to give you a temperature of 45°C. You can watch the GS loop inlet and outlet temperature. For cooling the outlet temperature would increase. For heating the outlet temperature would decrease, because the inlet temperature is the temperature of the HP evaporator. The HP itself may stop when this temperature is at around 5°C, to prevent freezing. In a real system the auxiliary heater is turn on at this point. I do not know how your GSHE is defined yet. If you increase the ground temperature, the capacity should increase. Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: xing.shi.2006@...
> Date: Tue, 9 Aug 2011 23:44:52 +0000
> Subject: [EnergyPlus_Support] Re: Ground source heat pump hot water supply T
>
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>
>
> Thank you! A further question.
>
>
>
> We are a little confused by the Setpoint function in EnergyPlus. let's say we model a Heat Pump to supply heating to a fairly large zone with significant heating load. For whatever reason, we model the ground heat exchanger in a wrong way so that it does not have the enough capacity to draw heat from the soil in winter. We use the setpoint object to set the heat water supply temperature at 45 degree C. In this case, would the temperature drop from 45 because the HX does not have enough capacity or it always stays at 45 because we use setpoint to force it at 45?
>
>
>
> Thanks!
>
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@> wrote:
>
> >
>
> >
>
> > I have not looked at your IDF yet. In general, GSHP do not have large heating capacity. If the Winter ground temperature is still at 18°C. it should work well. You may try increasing the capacity of the heat pump and the GSHE. Replace the HP with District heating and see what is the heating capacity required.
>
> >
>
> > Dr. Li
>
> > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
>
> > From: xing.shi.2006@
>
> > Date: Tue, 9 Aug 2011 21:09:42 +0800
>
> > Subject: [EnergyPlus_Support] Ground source heat pump hot water supply T
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> > We are modeling a 5 story building with two chillers and one ground source heat pump. Two chillers and the heat pump provide cooling in summer and the heat pump alone provides heating in winter.
>
> > We tried to use the Controller:WaterCoil object to vary the heating coil supply water flow rate to change the heat provided to the zone. The heating supply water T is set to 45 degree C using SetpointManager:Scheduled and the node applied is HW Supply Outlet Node.
>
> > We did not get any severe error. However, in winter we found that the HOT WATER LOOP:Plant Loop OutletNode Temperature [C](Hourly) varied from 45 degree C down to quite low. We were not sure about why this happend.
>
> > Please help. Attached is the IDF file. Thanks! XS
>
> >
>
__._,_.___
Primary EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx
The primary EnergyPlus web site is found at:
http://www.energyplus.gov
The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/
Attachments are currently allowed but be mindful that not everyone has a high speed connection. Limit attachments to small files.
EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
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