Estimation Method of Energy Consumption of Hot Water Radiant Heating System

Transcription

1 Estimation Method of Energy Consumption of Hot Water Radiant Heating System H. Miura 1, T. Sawachi 2, Y. Hori 3 and A. Hosoi 4 1 Department of Environmental Engineering, Building Research Institute, Japan 2 Building Department, National Institute for Land and Infrastructure Management, Japan 3 Faculty of Art and Design, University of Toyama, 4 Prefectural University of Kumamoto miura@kenken.go.jp SUMMARY The purpose of this paper is to develop the calculation method of energy consumption for hot water radiant heating system. The method was developed based on 4 experiments which were carried out in a wooden house built in an environmental chamber in the Building Research Institute (BRI). The method was steady state calculation for simplicity, of which necessary input information was limited to the one we can get easily in design stage such as catalogue value and design plan, besides the heating load calculated by simulation. Therefore, by using this method, we can design the heating system or use for rating of the system for energy saving. The calculation results of the energy consumption were compared with the measured results and then it was shown that these results agreed well with various condition. INTRODUCTION Up to now, in Japan, energy performance of residential buildings for heating and cooling has been evaluated by the heat loss coefficient and air tightness of the envelope. On the other hand, as residential heating/cooling equipments such as room air conditioner and radiant heating system have become more efficient, design of these equipments as well as the envelope has become more important for energy savings. Therefore, energy calculation methods for these various equipments have been required in order to compare and rating the energy performance of various heating systems based on primary energy consumption. In this paper, a calculation method of hot water radiant heating system is presented. OUTLINE OF THE CALCULATION METHOD Annual energy consumption of hot water radiant heating system can be calculated by using the method presented here. Before this energy calculation, heating load needs to be calculated by simulations. Necessary input information besides the heating load was limited to the one we can get easily in design stage such as catalogue value and design plan (Figure 1). DEVELOPMENT OF THE CALCULATION METHOD Outline The method was developed on the basis of experiments. Various hot water radiant heating systems, of which components such as radiator, boiler, and hot water circulating piping were

2 different, were constructed in a wooden house built in an environmental chamber in the Building Research Institute (BRI), and then heat loss and heat supply from the components were measured under various heat loads by changing the thermal insulation performance of the envelope, ventilation rate and external temperature. The method was based on the steadystate data for simplicity although heat flows and temperatures are usually changing. Therefore, experimental room was controlled to be steady state and then the data was used when all temperatures were constant. Climate condition Heat load calculation On-off pattern for heating Envelope performance Heat load Type and spec of the radiator Thermal performance of the piping Annual energy consumption Efficiency of the boiler Calculation method of energy consumption mentioned in this paper Figure 1. Necessary Input Information for Calculation Method Experiment 1) Experimental House A radiant floor panel was constructed at the 1 st floor of a two-story wooden house (Fig. 2 and 3) in a chamber which temperature and humidity could be controlled. This wooden house is equipped with central ventilation system by which air supply and exhaust rate can be controlled. Thickness of the insulation at the floor and the wall can be changed. Figure 2. Floor Plan Figure 3. Elevation View 2) Floor Heating System A normal gas boiler and a latent heat recovery gas boiler were tested, which were used for both floor heating and hot water supply. Supply water temperature of the normal gas boiler

3 for heating are 6 o C and that of the latent heat recovery gas boiler are 4 or 6 o C which can be selected with the controller. Even if 4 o C were selected, supply water temperature is controlled to be 6 o C when room temperature does not reach set temperature. Output of both boilers for heating is 2.56 ~ 14. (kw). Two radiant floor panels were installed in the living room and the kitchen (Fig.2). Figure 4 shows the cross section of the panel. Hot water piping consisted of two tubes for supply and return water which were bound up. Two types of the piping were used, which were insulated type and non-insulated type. The heat loss coefficient of non-insulated type was.21 (W/mK) and one of insulated type was.16 (W/mK). The length of the piping between the boiler and the panels was 1m. Figure 4. Cross Section of Radiant Floor Panel 3) Parameters of the experiments The experimental parameters shown in Figure 5 were 1) type of the boiler and hot water supply temperature, 2) type of the piping, 3) ambient (chamber) temperature and 4) thermal performance of wooden house which was established with changing thickness of the floor insulation and ventilation rate of the rooms. With combining these parameters, 4 experiments were carried out. Figure 5. Combination of Experimental Patterns * Heat loss coefficient per floor area of level high was 4.2 (W/m 2 K) with ventilation rate 17.5 (m 3 /h) and that of level low was 2.7 (W/m 2 K) with ventilation rate 63(m 3 /h). 4) Time of the operation and the measurement The floor heating was operated for 6~11 hours until the temperatures of the floor and the panel became constant. The measurement was started an hour before the start of the operation and stopped 3 hours after the operation was stopped in order to observe the decrease of the temperatures. 5) Experimental results Among 4 experiments, one result is shown in Figure 6, which was carried out under the situation that the boiler was normal gas boiler, the piping was non-insulated and the external temperature was 5 o C. Supply water temperature was kept about 75 o C for 6 minutes after

4 starting the operation to raise the floor surface temperature rapidly. The water from the boiler was intermittently supplied at about 6 o C and the cycle of on-off operation was 2 minutes. For minutes, water was supplied for 15 minutes and stopped for five minutes as well as combustion of the gas boiler. After 17 minutes when the room temperature reached the set temperature 2 o C, the operation time shortened gradually. The floor surface temperature was constant at about 31 o C and moved somewhat up and down by on-off of the water supply. The supply water temperature at the input of the panel was approximately 3 o C lower than that at the output of the boiler and the decrease of the supply water temperature was shown. On the other hand, the decrease of the return water temperature was not found because the return water seemed to receive the heat from the supply water, which temperature was higher than that of the return water, in the pair tubes. Temperature ( o C) Surface on the piping Surface on the floor Room Surface below the floor Supply at the boiler Supply at the panel Return at the panel Return at the boiler 1 Crawl space External Flow rate (l/min) Gas consumption Hot water supply Figure 6. Experimental Result Heat supply to the room, heat loss from the panel, heat loss from the piping and heat loss from the boiler which were defined in Figure 7, were calculated. The results were shown in Figure 8. The heat supply to the room as well as the heat loss from the piping increased as the external temperature lowered. The heat supply was influenced by the external temperature, e.g. 3~7 (W) with the external temperature 15 o C and 15~2 (W) with the external temperature -5 o C. On the other hand, heat loss from the boiler was not influenced well by the external temperature and heat supply to the room, and it was constant at 5~7 (W) at the normal gas boiler and at 25~4 (W) at the latent heat recovery gas boiler. Figure 7. Definition of Heat Supply and Heat Losses

5 4 Heat supply and loss (W) Heat loss from the boiler Heat loss from the piping Heat loss from the panel Heat supply to the room 5 Type of N L N L N L N L N L N L N L L N L L N L N L N L N L N L L N L L N L L N L L N L N L L the boiler * Type of the piping *2 N I N I N I N I N I N I N I N I N I External temp. -5 o C o C 5 o C 1 o C -5 o C o C 5 o C 1 o C 15 o C Level of thermal performance of the room Low (Thickness of the insulation = 4mm) High (Thickness of the insulation = 9mm) *1 N6:Normal gas boiler *2 N:Non-insulated piping L6:Latent heat recovery gas boiler of which supply temperature was 6 o C I:Insulated piping L4:Latent heat recovery gas boiler of which supply temperature was 4 o C Figure 8. Heat Supply and Heat Loss Development of the estimation method of the energy consumption Energy consumption (E) of the boiler is given by, 1 E = ( qpnl + qpipe ), (1) ehs where: e hs efficiency of the boiler; q pnl heat supply to the panel in W; q pipe heat loss from the piping in W. Heat supply to the panel is given by, Tpnl Troom Tpnl Tcrawl qpnl = qpnl, spy + qpnl, loss = + Afp (2) Ru Rd where: q pnl,spy heat supply from the panel to the room in W; q pnl,loss heat loss from the panel in W; T room room temperature in o C; Tcrawl crawl space temperature in o C; R u thermal resistance between the panel and room in m 2 K/W; R d thermal resistance between the panel and crawl space in m 2 K/W; A fp area of radiant floor panel in m 2. Equation (2) can be modified without using T pnl as follows; 1 qpnl = qpnl, spy + ( qpnl, spy Ru + Troom Tcrawl ) Rd (3) Ru + R d Troom T crawl = qpnl, spy + Rd Ru + Rd In the parentheses in Equation 3, the first term represents heat supply from the panel to the room and this equals the loss from the room to the outside through the envelope except the floor (L env ). The second term represents heat loss from the room to the crawl space through the floor (L pnl ) when floor heating is not operated. Therefore, sum of the first term and the second term equals to the heat load (L) calculated heat load calculation and Equation 3 can be modified as follows;

6 R + R q = L + L ( ) u d pnl env pnl Rd Ru + Rd = L (4) Rd The calculated results of the heat supply to the panel as mentioned above were compared with the measured results in Figure 9. The calculated results agree well with the measured results. Measured Value (W) Calculated Value (W) Figure 9. Comparison of q pnl Heat loss from the piping is given by, qpipe = Kpipe ( Twater, ctrl Tcrawl ) d lpipe (5) where: K pipe thermal coefficient of the piping in W/mK, T water,ctrl set temperature of supply water in o C, T crawl crawl space temperature in o C, d decreasing rate of average water temperature, l pipe length of the piping in m. Decreasing rate of the average water temperature (d) is the coefficient which represents that average heat loss decreases when on-off operation because water temperature decreases when water supply stops and which is given by, ( K pipeh( 1 r) c) 1 e d = r+ (6) Kpipeh c where: r operating rate which is the ratio of the water supply period, c thermal capacity of the water and pipes in J/mK h on-off operation cycle in s. It is difficult to define operating rate (r) qualitatively because it depends on how to control the boiler. In this paper, r was calculated from the heat supply to the panel q pnl conveniently assuming that r is relative to q pnl which shows in Figure 1. Figure 11 shows comparison between the calculated heat loss from the piping and measured one. Calculated results do not agree well with the measured one. The difference between the average water temperature and the crawl space temperature could be explained well, so the assumption that Kpipe was constant did not seem to be relevant. Figure 12 shows the measured efficiency of the boiler. When the output was higher than the lower limit of the boiler 2.56kW, the boiler ran continuously and the efficiency was constant. On the other hand, when the output was lower than 2.56kW, the boiler ran intermittently and the efficiency decreases. In this report, in case the output is lower than the lower limit, approximately curve of the efficiency was made on the assumption that heat loss was constant as below.

7 Heat supply to the panel ( qpnl ) (W) Floor Insulation, Controlled Water Temperature 4mm, 4 degree C 4mm, 6 degree C 9mm, 4 degree C 9mm, 6 degree C Operation rate ( r ) Figure 1. Relation between q pnl and r Experimental Results (W Calculated Results (W) Figure 11. Comparison of Heat Loss from Piping Table 1. Heat Loss from Boiler (which assumed in the calculation) Normal gas boiler with output water temperature 4 o C Latent heat recovery gas boiler with output water temperature 6 o C Latent heat recovery gas boiler with output water temperature 4 o C 853 (W) 64 (W) 32 (W) 1% 9% 8% Efficiency of the boiler (%) 7% 6% 5% 4% 3% Water supply was operated intermittently. (supply water temp. = 4 degreec) L6 L6 ( - 6min ) N6 N6 ( - 6min ) L4 2% 1% Water supply was operated intermittently. (supply water temp. = 6 degreec) Lower limit of the output when operating the boiler continuously = 256(W) % Burner was operated intermittently. Procedure of the calculation Water supply and burner were operated continuously Output of the boiler (W) Figure 12. Efficiency of Boiler Figure 13 shows the procedure to calculate the primary energy consumption from the heat load calculated by simulation and design value of the floor heating.

8 Figure 13. Procedure to Calculate Primary Energy Consumption Comparison of the calculated energy consumption with measured one Energy consumption which was calculated as mentioned above was compared measure one (Fig. 14). The calculated results agree well with the measured results. CONCLUSION The estimation method was developed based on the 4 experiments. The calculation results of the energy consumption agreed well with the measured one. This calculation needs input we can get by the heat load calculation and catalog value. Therefore, by using this method, we can decide which heating system should be used such as radiant heating, room air conditioner and so on, and in case of floor heating, we can decide the capacity of the boiler, type of the piping, thickness of the insulation of the floor and so on. This calculation can also be used for the rating of the heating system for energy saving. Experimental Results (W Calculated Results (W) Figure 14. Comparison of Primary Energy Consumption Energy consumption can not be calculated by this method a few minutes after the system on because the developed method was steady state calculation. In Japan, intermittent airconditioning is more popular than continuous air-conditioning. Therefore, it is necessary to develop the non steady state calculation of the energy consumption.