Vertical loop depth calculation

Discussion in 'Vertical and Horizontal Loops' started by youngy32, Oct 9, 2010.

  1. youngy32

    youngy32 New Member

    I am a mech eng student in Brisbane, AUS and am currently doing my thesis on GHPS. Part of my thesis is to come up with a conceptual design of GHPS for a "typical" office building around Australia. The typical office building I have found to be 10 stories with a total area of 11250m2 with limited area around the building. Because of the limited area and lack of information on depths of ground water i have chosen to go for a closed vertical loop heat exchanger. As the design is for a "Typical" office building I have used a simple load calculator which uses rule of thumb data (refer table 1)to determine the load of the building with inputs being the total area and occupancy of the building. I realise that rule of thumb data isnt the most accurate way of calculating the conditioning load of a building but as this is for an average office building i feel that this is acceptable. The calculator gives the results shown in table 2. I have then tried to find rule of thumb data for vertical loop depth and the most common recommendation is for every one ton of cooling a bore hole depth of 61m is required. As the cooling load is dominant in this case i have used the 384.6ton of cooling. This means that to cover this cooling load i would need approx 23460m of borehole depth and with a typical depth of each bore being 100m, a total of 234 bores would be required. This seems to me to be a little outrageous (oversized). Are my calcs wrong? or is this application just not viable? I have also attached a draft of my report so far, it is a little rough right now but has a little more info than what i have mentioned here. can anyone help me please!!!!?????

    Table 1:inputs
    @font-face { font-family: "Times New Roman"; }@font-face { font-family: "Arial"; }@font-face { font-family: "宋体"; }@font-face { font-family: "Calibri"; }@font-face { font-family: "SimSun"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm 0cm 10pt; line-height: 115%; font-size: 11pt; font-family: Calibri; }table.MsoNormalTable { font-size: 10pt; font-family: "Times New Roman"; }table.LightListAccent5 { font-size: 10pt; font-family: Calibri; }div.Section1 { page: Section1; } Load Type
    Average Value
    Average Cooling Load
    110.25 W/m2
    Average Cooling airflow
    5.08 L/s/m2
    Average Ventilation
    Average Ventilation per person
    Average Heating Load
    102.375 W/m2
    Table2: results@font-face { font-family: "Times New Roman"; }@font-face { font-family: "Arial"; }@font-face { font-family: "宋体"; }@font-face { font-family: "Calibri"; }@font-face { font-family: "SimSun"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm 0cm 10pt; line-height: 115%; font-size: 11pt; font-family: Calibri; }table.MsoNormalTable { font-size: 10pt; font-family: "Times New Roman"; }table.LightListAccent5 { font-size: 10pt; font-family: Calibri; }div.Section1 { page: Section1; } Load Type
    Total Cooling Load
    1,352,362.2 W
    Total Cooling Tonnage
    384.6 ton
    Total Cooling Airflow
    62,203.1 L/s
    Ventilation Airflow
    6,625.6 L/s
    Total Heating Load
    1,255,764.9 W

    Attached Files:

  2. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Sounds about right for the cooling load that you calculated.

    I have a spreadsheet with 30 case studies of commercial buildings.

    The combined cooling loads is 6342 tons, the combined bore hole depth is 1,144,970'

    This works out to an average of 180' of bore depth per ton.

    Some ground loops now are designed to meet the heating load with a cooling tower added to make up the additional load to meet the cooling needs.
  3. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    In the above case studies the most bored foot per ton was 305 and the least was 122.

    The largest project had 306 bore holes at 186 feet per ton.

    the smallest project had 2 boreholes at 167 feet per ton.
  4. youngy32

    youngy32 New Member

    The calcs are correct then, i'm surprised. I think i will have to go for the hybrid system with an attached cooling tower and plate heat exchanger to make up the difference between my heating and cooling load. Although my heating load is pretty close to my cooling load requirements i may also have to ammend my heating load input (W/m2) as i have seen other office loads in a similar tropical environments to AUS where the heating load is usually 1/2 to 3/4 of the cooling load. Especially here in Brisbane were heating is minimal and would usually only be required for only really 2-3 months of the year as it doesn't get too cold here. Amending this input would also allow me to avoid adding a boiler into the system as the heating load i have now still would not be able to be covered because the number of boreholes required with around 15-20ft separation would not fit within the footprint of my building. cheers for your input palacegeo, appreciate it.
  5. youngy32

    youngy32 New Member

    also is sizing my cooling tower as simple as just making up the difference in load.
    eg: say heating load is 720kW and cooling load is 1352kW the cooling tower would be rated at around 632kW??
  6. SoundGT

    SoundGT New Member

    Rule of thumb is a dangerous way to model a commercial GLHE. Palace is correct with his variations – and there is a reason for it. Earth conditions and weather conditions are highly variable. You need the heating load & cooling load (preferably in 8,760-hours/year format), thermal conductivity, diffusivity, deep earth temperature, etc. in order to correctly calculate the size of the GLHE.
    We design city systems on a regular basis. The first thing we look for is ground or subsurface water. That not available we start looking at space, inclusive of the building footprint. If sufficient area is not available we begin to look at a hybrid system that will reduce the amount of loop required and/or other places to move energy – gray water, black water, solar thermal – depends on the application..
    Re: Hybrids. Across the US the it is generally the case that 50% of the building load will cover 90% of the heating/cooling needs for the year. Suggest that you look at a bin data profile to validate this assumption in your area.
    There are a number of very good references available. See Kavenaugh & Rafferty “Design of Geothermal Systems for Commercial and Institutional Buildings” this is the commercial GX bible.
    Also see ASHRAE handbook HVAC Applications, Chapter 32. – Also, numerous papers are available from ASHRAE.
    For hybrid systems, one of the best studies is Scott Hackel’s research paper. – Available from ASHRAE, “Development of Design Guidelines for Hybrid Ground-Coupled Heat Pump Systems ASHRAE TRP-1384”
  7. SoundGT

    SoundGT New Member

    Hybrid Systems

    Do not confuse peak loads with total consumption. There are a number of places where the peak heating matches the peak cooling. How about the case where this is true but the duration of one of the peaks is very small? The total annual load for heating and cooling is more important. This is why you should use a program that calculates 8760 loads. There is also a very good paper on Equivalent Full Load Hours – Carlson & Thornton “Development of Equivalent Full Load Heating and Cooling Hours for GCHPs”

    Size of hybrid equipment: the difference between the peak loads is a starting point. If you are adventurous, you can use the diversified peak loads, which are generally less than the calculated peak block loads. With a hybrid system, balancing the ground loads is important so not only the size of the hybrid equipment is important but also the control strategy is important.

    Re: Ground Load. This is an gross simplification - not withstanding the people/lights/equipment contribution , in the heating season approximately two thirds of the energy to heat the building comes from the ground the balance comes from compressor energy, fan energy, and pumps. In the cooling season, you dump all of the building load to the ground plus the compressor energy, fan energy, and pumps.
  8. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader


    This is shown in the above graph.

    Notice how the EWT closely tracks the ground temp during the heating season, but jumps way above it during the cooling season.
  9. SoundGT

    SoundGT New Member

    Good example Dewayne. - Thanks
  10. Drew

    Drew New Member

    GLHEpro vertical loop sizing

    I am a masters student and am having problems sizing my GSHP system using GLHEpro. My system is to service 6 buildings on my university campus. I wish to find various system loop sizes; initially to service the building without any auxiliary heating/cooling equipment and then using a hybrid system to greater and lesser degrees.
    I initially tried entering the total heating/cooling loads into GLHEpro (as shown below ‘Total Heating’ and ‘Total Cooling’) and left all of the default settings the same. I set the borehole spacing to the biggest I could find (10*12). However, when I tried GLHEsize I was firstly warned that my system would require a fluid flow rate of 61.124 L/s which is many times higher than the default of 1.994 L/s. Even though I didn’t understand this I entered it and then tried the sizing again and then was warned that the ‘borehole spacing to depth ratio was lower than the smallest data point for the G-function’ and that I may want to revise my system.
    I thought that maybe the total loads were too big for the software so I divided them all by 6 to roughly represent each building’s load and tried again. Again I received a warning that I would need a very high fluid rate (59.568 L/s) and after entering and trying again I got the same warning regarding borehole spacing to depth ratio as above.
    I’m obviously doing something very wrong but am at a loss as to what it is. I must get this software to work for me asap as my thesis needs to be completed in two weeks.
    Can anyone please advise me on where I’ve gone wrong and how I can correct it?
    Total heating
    Total Cooling

  11. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    i received an email from Drew giving the same data, except in the email, the heating and cooling loads are given in kWh-e units. What is the conversion factor to Btur/H?
  12. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    1 kWH = 3412.3 BTU/hr
  13. Drew

    Drew New Member

    Hi guys
    Thanks for looking at my thread. You brought up the loads were in kWh not Btu. I thought the conversion was 3414 Btu to 1kWh? I revised the loads as I think they were off and have pasted them again below.
    The sizing program GLHEpro i entered the figures for total heating and cooling and it said that with 10*12 borehole grid and a spacing of 30ft, I would need boreholes several thousands of ft deep. I then tried entering the figures for just one building with the same configuration and it said my boreholes would still be around 1000ft. As an experiment I extended the borehole spacing to 100ft and then it said that the required borehole spacings would be around 500ft. Obviously 100 ft spacing between vertical bores is not viable so I think i must be going seriously wrong somewhere.

    Anyone have any ideas?
    Last edited: Dec 7, 2010

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