Energy and cost associated with ventilating office buildings in a tropical climate

Abstract

Providing sufficient amounts of outdoor air to occupants is a critical building function for supporting occupant health, well-being and productivity. In tropical climates, high ventilation rates require substantial amounts of energy to cool and dehumidify supply air. This study evaluates the energy consumption and associated cost for thermally conditioning outdoor air provided for building ventilation in tropical climates, considering Singapore as an example locale. We investigated the influence on energy consumption and cost of the following factors: outdoor air temperature and humidity, ventilation rate (L/s per person), indoor air temperature and humidity, air conditioning system coefficient of performance (COP), and cost of electricity. Results show that dehumidification of outdoor air accounts for more than 80% of the energy needed for building ventilation in Singapore's tropical climate. Improved system performance and/or a small increase in the indoor temperature set point would permit relatively large ventilation rates (such as 25 L/s per person) at modest or no cost increment. Overall, even in a thermally demanding tropical climate, the energy cost associated with increasing ventilation rate up to 25 L/s per person is less than 1% of the wages of an office worker in an advanced economy like Singapore's. This result implies that the benefits of increasing outdoor air ventilation rate up to 25 L/s per person--which is suggested to provide for productivity increases, lower sick building syndrome symptom prevalence, and reduced sick leave--can be much larger than the incremental cost of ventilation.

Fig 1. Annual outdoor weather conditions for Singapore.

(A) dry-bulb air temperature (°C). (B) humidity ratio (in units of g of H₂O vapor per kg of dry air). (C) specific enthalpy (kJ per kg of dry air).

Fig 2. Enthalpy of outdoor and indoor air in Singapore.

(A) Enthalpy of moist air for outdoor and indoor conditions. The plotted outdoor condition represents the daily average for building operating hours (M-F 7 am to 7 pm); the indoor condition corresponds to temperature = 23°C, RH = 50%. (B) Monthly sensible and latent energy needs to condition air at a ventilation rate of 10 L/s per person for building operation hours (M-F 7 am to 7 pm).

Fig 3. Baseline ventilation energy requirements and cost in Singapore.

(A) Annual ventilation energy consumption (kWh/y per person) in relation to the indoor temperature and RH set point to provide 10 L/s per person with a COP = 4 during building operating hours, weekdays (M-F) from 7 am to 7 pm. (B) Annual electricity cost (US dollar ($)/y per person) for providing 10 L/s per person. (Analysis uses an electricity cost of 0.26 Singapore dollars per kWh, valid as of September 2014; Source: Singapore Power Tariff Rates [46]). Note: US$1 = S$1.25 as of September 2014.

Fig 4. Ventilation energy requirements and cost in Singapore in relation to ventilation rate.

(A) Annual cost (US$) and energy use with thermal conditioning of ventilation air for varied indoor set-point temperature (with COP = 4 and RH = 50%). (B) Annual cost and energy use associated with thermal conditioning of ventilation air for different COP values (temperature = 25°C and RH = 50%). Electricity cost assumed to be US$0.21 (= S$0.26) per kWh. Calculations are based on operating the ventilation system from 7 am to 7 pm on all weekdays (M-F) of the year.

Fig 5. Cost of conditioning ventilation air in Singapore in relation to air conditioning system performance and electricity price.

Effect of COP and the unit cost of electricity on ventilation energy cost (US$/y per person) assuming a set point temperature = 25°C and RH = 50%. (A) Ventilation rate = 10 L/s per person and (B) Ventilation rate = 25 L/s per person. The analysis is based on an office-building operating schedule (7 am to 7 pm all weekdays of the year, Monday-Friday).