LIFE CYCLE COSTS FOR INDOOR CLIMATE SYSTEMS WITH REGARDS TO SYSTEM CHOICE, AIRFLOW RATE AND PRODUCTIVITY IN OFFICES

Transcription

1 LIFE CYCLE COSTS FOR INDOOR CLIMATE SYSTEMS WITH REGARDS TO SYSTEM CHOICE, AIRFLOW RATE AND PRODUCTIVITY IN OFFICES Dennis Johansson 1,2 1 Research and Development, Swegon AB, Tomelilla, Sweden 2 Building Physics, Lund University, Lund, Sweden ABSTRACT Assumptions of productivity costs related to the outdoor supply airflow rate and indoor temperature can be made based on a number of recent studies. A life cycle cost (LCC) computer program for indoor climate systems based on Swedish conditions was developed and used to compare and optimize different indoor climate systems. A productivity cost related to the outdoor supply airflow rate and the indoor temperature according to the recent studies was assumed. A higher outdoor supply airflow rate, as well as a more correct indoor temperature, increases the energy use and in some cases the designed maximum power and thereby the initial cost of the indoor climate system, which includes the heating, ventilation and cooling systems. The results indicate that a much higher outdoor supply airflow rate at presence is appropriate from a life cycle cost perspective as well as a cooling system is, even if the influence on the human being still should be taken with precaution. With that higher airflow rate, the results show that there is a clear benefit with variable airflow rate ventilation systems due to the low reported occupancy level and the option, at absence, to lower the airflow rate a lot. KEYWORDS LCC, life cycle cost, indoor climate systems, productivity, energy INTRODUCTION People spend up to 9% of their time indoors (Lech et al., 1996). To ensure people s health and comfort when they are indoors, the indoor air quality and thermal comfort must be set at an appropriate level. An indoor climate system, consisting of heating, cooling and ventilation, serves this purpose (Nilsson, 23). The aim of the indoor climate system is to provide the building with a good thermal comfort and indoor air quality. A number of studies have indicated a significant relationship between the ventilation airflow rate and health and productivity in regards to offices, schools and dwellings. In this study, the life cycle cost (LCC) of the indoor climate system, which means heating, cooling and ventilation, was simulated for a theoretical office building. Different ventilations systems were simulated resulting in different heating and cooling system designs with the computer program ProLive based on (Johansson, 25). A health related cost was added to the life cycle cost dependent on the airflow rate and the indoor temperature respectively. A higher airflow rate or a smaller acceptable indoor temperature span results in a more expensive indoor climate system both in regards to the life cycle cost and the initial cost. No comprehensive literature review was done regarding the cost and health impacts from the ventilation airflow rate. Only a theoretical building has been simulated, a typical cell office building. Field measurements were not an alternative, since it would be difficult to measure the life cycle cost, particularly for different airflow rates. Only mechanical ventilation systems were included. Corresponding Author: Tel: address: dennis.johansson@swegon.se

2 METHODS One problem with the design of an indoor climate system is that there has been a predominant focus on initial costs. A life cycle approach could improve both the energy and economic performance of the indoor climate system. The life cycle cost of a product is the sum of all costs related to that product over its entire life span. Future costs are discounted to the value of today, the net present value, by the use of a discount rate of interest. The computer program ProLive was developed to calculate the life cycle costs for indoor climate systems in offices, schools and dwellings (Johansson, 25). It handles thousands of combinations of heating, cooling and ventilation systems typically used in Sweden. It also takes into account the initial costs for buying and mounting components through a power demand calculation, energy costs, maintenance costs, repair costs and costs for space loss due to system components. Costs are based on Swedish prices from Sektionsfakta (Wikells, 23), which is a popular cost database for the building sector. Outdoor climate data was obtained from the computer program Meteonorm (Metotest, 23), which simulates outdoor climate data for the entire world. ProLive has features to model productivity and health costs based on indoor temperature and airflow rate even if the user has to provide the correlation data between the parameters and the costs. ProLive uses Swedish costs in SEK excluding VAT (25% value added tax). 1 SEK.14 US$.11 as of In this study, the price of heat was set to.6 SEK/kWh and the price of electricity was set to.8 SEK/kWh excluding VAT. The discount interest rate was assumed to be 1% for electricity, 2% for heat and 3% for other costs representing a real price increase for heat and even more for electricity. An annual value of 1 SEK/m² was assumed for space loss. It was assumed that there was a 2% deduction from the initial costs for both air and hydronic components compared to Wikells (23). A 4 year calculation period was used. The scrap value was assumed to be zero. Buildings The simulated building was a theoretical one-corridor four storey cell office building with the corridor in the south-north direction and office cells on each side of the corridor. On each storey, there were 2 office cells. Assumed data for the building is given in Table 1. The outdoor climate data were from Stockholm, Sweden. The building had a medium weight interior construction. Table 1. Assumed data for the simulated building. Storey height 3 m Heat plant eff. 9% Cell length 3 m Chiller COP 2 Building length 3 m Room temperature min 21 C; max 24 C Corridor width 3 m Internal load presence 25 W/m² Cell width 3 m Internal load abscence 2 W/m² Total floor area 18 m² Occupancy daytime 5% Heat transmission area 19 m²/room Occupancy other time 5% Heat ttransmittance.5 W/(m² K) Ventilation air flow rate.35 l/(s m²)+7 l/person Duct pressure drop 1 Pa/m Solar rad. transmittance.4 Supply air temperature 18 C Leakage at 5 Pa.8 l/(s m²) Heat recovery temp. eff. 8% Window area 1m²/room Indoor climate systems The default indoor climate system was a supply and exhaust ventilation system with constant airflow rate, passive beams for support cooling and hydronic radiators for support heating. The air was supplied by ceiling diffusers and extracted from each cell. The duct sizes were allowed to vary within the branches and main duct respectively. With this the set up, one zone was used. It was assumed that

3 the office was designed for one person in each cell. Daytime was supposed to be between 8: and 18: five days a week. In the default case, no productivity related costs were included. The typical airflow rate in a Swedish office building is.35 l/(s m²) + 7 l/(s person) based on Boverket (22) and Enberg (1996). This was used as default airflow rate in the study. The alternative systems had an occupancy controlled airflow rate, a timer controlled airflow rate, a temperature controlled airflow rate, different duct system layouts, without cooling and with different supply diffusers. Productivity costs Based on recent studies collected by Seppänen and Fisk (25), Johansson (25) presented a model for productivity as a function of airflow rate depending on the combination of short term sick leave and productivity related airflow rate. Equation 1 gives the relative decrease in productivity, PI q, where q is the airflow rate in l/(s person). q = 6.5 PI q e (1) Seppänen and Fisk (25) also looked at productivity related to temperature based on a weighting of 26 studies. Johansson (25) simplified their relationship to Equation 2 which is the same curve form that was used by Jönsson (25), where PD t is the relative decrease in productivity with the room temperature as t room in C. ( t 21.6) 2 PD.155 (2) t = room These productivity costs were included respectively in parts of this study. They were calculated hourly and scaled by the occupancy level at each hour. RESULTS Figure 1 gives the life cycle cost for the simulated office building split into components and parts as a function of the airflow rate, q, for the default constant airflow rate (CAV) system and for a demand controlled system with chilled beams for cooling. Space loss is the space that is taken up by ducts and air handling unit. At some points the cost increases in large steps due to the available component sizes on the market. The default airflow rate is 1.1 l/(s m²) based on.35 l/(s m²) + 7 l/(s person) and an office cell area of 9 m². In Figures 1-3, there is no included productivity related cost. Table 2 presents the life cycle costs for different indoor climate systems. Some systems do not have cooling and should not be compared with the default system from an indoor climate perspective. In system number 13, there is no cooling but a temperature based productivity cost is added. System number 12 gives the result for the simulated building for only one floor, which means that the floor area is one fourth of the default set up. Systems number 8 and 9 uses outdoor climate data from Los Angeles, United States, and Karasjok, northern Norway, respectively. By default, the daytime occupancy was 5% while the non daytime occupancy was 5%. In Figure 2, the overall average occupancy was varied from 3.5% to 1%. The 3.5% corresponds to a non daytime occupancy level of 5% and a daytime occupancy level of %. First, the daytime occupancy was increased to 1%, which means an average occupancy of 33%. Then, with the daytime occupancy was set to 1%, the non daytime occupancy was increased from 5% to 1%. This was done for two different airflow rates per office cell, 1.15 l/s, which was default, and 3 l/s.

4 Life cycle cost / (SEK/m²) q / (l/(s m²)) Life cycle cost / (SEK/m²) q / (l/(s m²)) Heat Chiller electricity Fan energy Space loss Repair Maintenance Chiller District heat exch. Air handling unit Adjustment Control Fire dampers Pipes, cold Chilled beams Pipes, heat Radiators Diffusers Silencers Exhaust duct comp. Exhaust ducts Supply duct comp. Supply ducts Figure 1. Life cycle cost (LCC) per total floor area over 4 years for a constant airflow rate (CAV) ventilation system to the left and for a demand controlled ventilation (DCV) system to the right. The cost is in SEK excluding VAT. Duct components means T-junctions, dampers, bends and reductions while ducts are a particular subgroup. The border between initial and running costs goes between Chiller and Maintenance.

5 Table 2. Life cycle costs per total floor area in SEK/m² for different indoor climate systems. Not all systems give the same indoor climate and should not be compared. CAV CAV with timer VAV cooling with air DCV occ. controlled with air cooling only CAV exhaust in corr., cons. DCV occ. controlled with chilled beams CAV no cooling CAV in Karasjok CAV in Los Angeles CAV induction unit below windows CAV low speed diffusers CAV 1 floor building CAV no cooling with salary 2 SEK/h Nbr Initial Fan el Heat Chiller el Maintenance Repair Space loss Prod. Loss 2915 LCC CAV exhaust only LCC / (SEK/m²) DCV 3 DCV 1.15 CAV 3 CAV timer 3 CAV timer 1.15 CAV Occupancy/% Figure 2. Life cycle cost (LCC) as a function of occupancy level for the default constant airflow rate ventilation system (CAV 1.15), for a constant airflow rate ventilation system with timer decreasing the airflow rate during the evening, which means that there should be no people in the building except during the daytime, and for a occupancy controlled airflow rate ventilation system (DCV 1.15). All three systems were also simulated with higher airflow rate per cell (3 l/s). The life cycle cost as a function of the life span, the accumulated life cycle cost, is shown in Figure 3. Every 15:th year, ProLive assumes that electronic and motorized equipment is supposed to be changed. After 4 years, it is assumed that the other equipment is changed, but in this case, it never happens.

6 Acc. LCC / (SEK/m²) CAV E DCV CAV CAV timer Time/years Figure 3. Accumulated life cycle cost (LCC) over time for different ventilation systems. CAV is the default system. CAV timer includes a timer that decreases the airflow rate during the evening. DCV means occupancy controlled airflow rate and CAV E, constant airflow rate with exhaust system only. Supply air comes from air inlets at the windows. LCC / (SEK/m²) Initial cost / (SEK/m²) q/(l/(s m²)) CAV timer - LCC CAV - LCC DCV - LCC DCV - Init CAV timer - Init. CAV - Init. Figure 4. Life cycle cost (LCC) and initial cost including a productivity related cost depending on the airflow rate with a salary of 2 SEK/h for three different ventilation systems. The systems are the same as in Figure 3. In Figures 1-3, no productivity related cost was included. Figure 4 shows the life cycle cost as a function of the airflow rate including a productivity related cost dependent on the airflow rate. This productivity cost is corrected to be zero at the highest presented airflow rate for the system that gives the highest productivity cost. Figure 4 also shows the initial cost of purchasing and mounting the indoor climate system. The salary cost was set to 2 SEK/h. The life cycle cost has an optimum while the initial cost increases with higher airflow rate.

7 At high enough salary levels, there is an optimal airflow rate if a productivity cost dependent on the airflow rate is included. Figure 5 shows the optimal airflow rate per cell floor area as a function of the salary cost. The optimal airflow rate is higher for the demand controlled ventilation system compared to the constant airflow rate ventilation system. This is because the higher airflow rate only is on at when there is people in the office cell for the demand controlled ventilation system. Optimal airflow rate / (l/(s m²)) Salary / (SEK/h) CAV DCV Figure 5. The optimal airflow rate per office cell floor area if a productivity related cost, dependent on the airflow rate, is included. CAV means the default constant airflow rate ventilation system while DCV means an occupancy controlled airflow rate ventilation system. If the productivity related cost is based on the indoor temperature and not on the airflow rate, the simulated life cycle cost can be described by Figure 6. Here, it is assumed a room temperature of 21.6 C with an allowed increase and decrease span given by the x-axis. A temp span of 5 C means that it is allowed to be between 16.6 C and 26.6 C. This decreases the need for heat and cooling and also increases the possible heat storage in the building interior. LCC / (SEK/m²) CAV 2 CAV 5 CAV Temp span above and below 21.6 C/ C Figure 6. Life cycle cost (LCC) if a productivity related cost is added based on the room temperature at different salaries,, 5 and 2 SEK/h.

8 DISCUSSIONS AND CONCLUSIONS The presented simulated life cycle costs for different indoor climate systems in the theoretical office building show that it is more expensive with variable and demand controlled ventilation systems. On the other hand, the prices were taken from 23 and there has been a remarkable decrease regarding electronics and motorized equipment which is beneficial for the demand controlled systems that also decrease the energy use. It has also been shown by Johansson (25) that in schools, the demand controlled systems can be beneficial even with the higher, out-of-date, price level. Therefore, the result should not be that there is no use for demand controlled systems. Johansson (27) has also shown that demand controlled ventilation is beneficial in dwellings. The future prices on different kinds of energy will also influence the system choice and the optimal airflow rate. If the energy price will increase faster than other prices, the demand controlled systems will be more beneficial. Through more research, it may be possible to show that the indicated benefits from a better indoor climate would have a great impact on the LCC of an indoor climate system. If cooling influences the productivity according to the recent studies mentioned and if the studies regarding airflow rate and productivity are correct, it should be beneficial to have a much higher airflow rate than the one usually used in Sweden. However, the resulting optimal airflow rate is so high that other criteria should limit the airflow rate. The optimal temperature span related to higher salaries is low and it can be argued that it is not possible to control the room temperature in such a precise way. It is reasonable to believe that the human being does not want exactly the same temperature over time. The result could be interpreted as the need for demand controlled temperature. Still, more research in this field is needed. There are still many questions and uncertainties regarding long term and combined effects. ACKNOWLEDGEMENTS This research and paper is financed by Swegon AB, Sweden, and initiated by Building Services and Building Physics at Lund University. REFERENCES 1. Boverket (22), Boverkets byggregler BFS 1993:57 med ändringar till och med 22:19, Karlskrona, Sweden, in Swedish 2. Enberg, H. (1995), Minimikrav på luftväxling, Enköping, Sweden, in Swedish 3. Johansson, D. (25), Modelling Life Cycle Cost for Indoor Climate Systems, Doctoral thesis, Building Physics, Lund University, Lund, Sweden, Report TVBH-114, 4. Johansson, D. (27), The cost of indoor climate systems in dwellings taking into account airflow rate, health and productivity, Proceedings of Clima 27, Helsinki, Finland 5. Jönsson, A. (25), Indoor temperature as a goods and as a factor of production, Proceedings of the 1th International Conference on Indoor Air Quality and Climate, Beijing, China, Lech, J.A., Wilby, K., McMullen, E., Laporte, K. (1996), The Canadian human activity pattern survey: Report of Methods and Population Surveyed, Chronic Diseases in Canada, 17, 3/4 7. Meteotest (23), Meteonorm handbook, manual and theoretical background, Switzerland, Nilsson, P.E., editor (23), Achieving the desired indoor climate, Studentlitteratur, Lund, Sweden 9. Seppänen, O., Fisk, W.J. (25), Some quantitative relations between indoor environmental quality and work performance and health, Proceedings of the 1th International Conference on Indoor Air Quality and Climate, Beijing, China, P-4 1. Wikells Byggberäkningar AB (23), Sektionsfakta-VVS, Wikells byggberäkningar AB, Växjö, Sweden, , in Swedish