Two buildings (“building A” and “building B”) are on adjacent sites. The project team wants to achieve two different LEED NC certifications. The buildings are heated by the same geothermal heat pumps, which are in building A. The heat pumps are 4. One produces hot water throughout the year (it's necessary to heat the domestic hot water), while the other ones produce heated water during the winter season and cooled water during the summer season. The document “Treatment of District or Campus Thermal Energy in LEED” states that “when the building housing the thermal energy plant is itself seeking LEED certification (…) Performance method: The district energy equipment shall be modeled as upstream equipment”. I would like to use option 2 “Aggregate Building / DES Scenario”.
1. Table 4 of the document “Treatment of District or Campus Thermal Energy in LEED” states that for the proposed model a “Virtual on-site hot water or steam boiler representing upstream DH system” shall be considered. At least for building A, wouldn't modeling the geothermal heat pumps explicitly be more accurate? Consider that the effective performance of heat pumps depends on the real conditions in which they work and also during the winter season the efficiency (COP) is higher than 1.
2. As for the baseline model, I don’t know if I understand correctly the Table 4 of the document “Treatment of District or Campus Thermal Energy in LEED”. I would follow the normal procedure: the conditioned floor area is 2882 m2, the conditioned floors are three, the heat is provided by electricity (heat pumps), therefore I would model System 6 - Packaged VAV with PFP boxes.
Any advice, please?
Best Regards
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 6, 2014 - 9:53 am
1. You would model a virtual well field (central plant) for each building. You would use the actual efficiency of the system as the efficiency for the system since there is no default value.
2. The baseline system is according to Table G3.1.1A.
Francesco Passerini
engineer90 thumbs up
May 6, 2014 - 10:57 am
Thank you. As for point 1., the real plant is composed by 42 boreholes and 4 reversible heat pumps. The heat pumps are connected in parallel. I'm going to model the two buildings separately because the projects have been registered separately. I think that I should consider the fact that for a single building the available heating/cooling rate is not the total heating/cooling rate of the plant (because at the same time also the other building needs energy). Moreover, the water temperature that comes back into the heat pumps and the part load ratio are influenced by the conditions in both buildings.
In the two virtual models, shall I consider only a part of the boreholes and only a part of the heat pumps?
The buildings have similar envelopes and they are both occupied by offices. Could I divide the “virtual boreholes” and the “virtual heat pumps” according to the envelope surfaces?
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
May 6, 2014 - 11:18 am
Hi Francesco, thankfully I've not had to do this myself yet, but from what I understand, you'll need to run simulations where both "buildings" (in energyplus/designbuilder these will all fall under the same building object) are connected to the plants. You then post process the total energy consumed by the shared plant and appropriate it to each building correctly. It's a damn headache...but I think it's the accepted way.
Since you can model everything working together, you could setup outputs at specific node points measuring the energy flow to each building. That might simplify things.
Francesco Passerini
engineer90 thumbs up
May 6, 2014 - 12:09 pm
Hello Jean, of course a model where both buildings are connected with the heat pumps is the most accurate way to model the real physical behaviour. Actually, if I create such a model, I will be tempted to use that model also for the official review. Would it be acceptable for the reviewers although the buildings are not part of the same project (they are registered separately)? As for the heating and cooling requirements, I could divide the electrical energy between the two buildings proportionally to the heating/cooling that the buildings need. Thank you.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 7, 2014 - 4:11 pm
I think either the "virtual" well field approach or what Jean suggests would both be accepted by the reviewer. In either case be sure to include a thorough narrative explanation and the calculations associated with any post-processing. I would proportion the virtual filed based on the capacity of the units within each building. So if building A is 50 tons and building B is 40 tons proportion the field accordingly. I don't think you should proportion the field based on envelope load alone since that is only a portion of the total load.
Francesco Passerini
engineer90 thumbs up
May 7, 2014 - 5:57 pm
All the heat pumps are in building A. Building B is connected with the heat pumps through pipes. I think that I'll proportion the field according to the total nominal power of heated floors and water convectors of the two buildings, or I'll follow the Jean's advice. Thanks!