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Probabilistic assessment of overcooling risk for a novel extra-low temperature dedicated outdoor air system for Hong Kong office buildings

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Abstract

Overcooling causes thermal discomfort and wastes energy. Effectiveness of a novel dedicated outdoor air system (DOAS) consisting of a multi-stage direct expansion coil to generate extra-low temperature (XT) outdoor air to handle the entire space cooling demand for better energy and humidity control has been confirmed in previous studies. In these studies, the terminal system was assumed using variable air volume (VAV) without reheating. The conventional VAV system without reheating has overcooling problem. This is associated with uncertain fluctuations of operating characteristics. However, virtually no study has been done to investigate its overcooling risk, and how it is compared with XT-DOAS. In this study, a rigorous probabilistic approach is proposed to account for the uncertain fluctuations of operating characteristics to assess their overcooling risks. Monte Carlo simulations with 10,000 interactions was used for risk analysis. A morphing method was employed to develop 10,000 weather files that could give an accurate prediction of future variations in yearly weather conditions. Reliable datasets were used to develop the probability distribution functions to account for the daily variations in use patterns. The most probable operating characteristics were input to EnergyPlus for hour-by-hour simulations. The annual cooling load profile formulated by the probabilistic approach was successfully validated by in-situ measurements. The probabilistic assessment results show that XT-DOAS has far less overcooling hours and lower long-term percentage of dissatisfied than the conventional VAV system. The results also indicate that the proposed probabilistic approach is useful for a robust system performance evaluation.

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Abbreviations

A :

heat transfer area for air side (m2)

ALD :

ASHRAE likelihood of dissatisfied

a :

empirical coefficient

b :

empirical coefficient

Con_VAV:

conventional variable air volume system

COP :

coefficient of performance of the chiller

C pa :

specific heat of moist air (kJ/(kg·°C))

C pw :

specific heat capacity of water (kJ/(kg·°C))

c :

empirical coefficient

DB:

dry bulb

DX:

direct expansion

dbt :

dry-bulb temperature (°C)

dpt :

dew point temperature (°C)

DSWF :

monthly mean global solar radiation (Wh/m2)

E :

electricity consumptions (kWh)

gsr :

global solar radiation (Wh/m2)

HKO:

Hong Kong Observatory

HR :

hour

h :

specific enthalpy (kJ/kg)

IA :

index of agreement

IAQ:

indoor air quality

k :

last progressive time step of the calculation period

LHS:

Latin hypercube sampling

LMTD :

log mean temperature difference between air and refrigerant (°C)

LPD :

long-term percentage of dissatisfied

MAE :

mean absolute error

MAFF :

minimum air flow fraction

MAPE :

mean absolute percentage error

MCS:

Monte Carlo simulation

m :

mass flow rate (kg/s)

OA:

outdoor air

OCC :

occupant

O :

observed value

Ō :

mean observed value

P :

predicted value

PDF:

probability distribution function

PLR :

part load ratio

Q :

cooling output/cooling load

RA:

re-circulated air

RH:

relative humidity

RMSE :

root-mean-squared error

SA:

supply air

SAT :

supply air temperature (°C)

SPHU :

percentage changes of specific humidity

s :

hourly specific humidity (g/kg)

T :

temperature (°C)

t :

time (hour)

TMY:

typical metrological year

TMAX :

maximum temperature (°C)

TEMP :

mean temperature (°C)

TMIN :

minimum temperature (°C)

V :

volume flow rate (m3/s)

VAV:

variable air volume

W :

chiller power input (kW)

x :

hourly weather variable

XT:

extra-low temperature

α :

fractional change/scaling factor

ρ :

density (kg/m3)

comf:

comfortable

e:

evaporating

in:

indoor

lat:

latent

m :

month

max:

maximum value

min:

minimum value

o:

baseline state

oc:

overcooling

op:

operative

sa:

supply air

sen:

sensible

spx:

space air

sw:

supply chilled water

t:

time

ra:

return air

rated:

rated condition

rw:

return chilled water

w:

chilled water

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Acknowledgements

This work is supported by the RGC Project No. PolyU 15208414.

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Authors and Affiliations

  1. Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China

    Yani Bao & W. L. Lee

  2. Department of Built Environment and Energy Utilization Engineering, Taiyuan University of Technology, Taiyuan, 030024, China

    Jie Jia

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  1. Yani Bao
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  2. W. L. Lee
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  3. Jie Jia
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Correspondence to W. L. Lee.

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Bao, Y., Lee, W.L. & Jia, J. Probabilistic assessment of overcooling risk for a novel extra-low temperature dedicated outdoor air system for Hong Kong office buildings. Build. Simul. 14, 633–648 (2021). https://doi.org/10.1007/s12273-020-0684-4

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