Experimental Study on Crawl-Space Heating with Thermal Storage using Heat Pump

Experimental Study on Crawl-Space Heating with Thermal Storage using Heat Pump Koji Fujita, Graduate Student, Graduate School of Science and Technology, Kobe University; 039d892n@stu.kobe-u.ac.jp Atsushi Iwamae, Assoc. Professor, Department of Architecture, School of Science and Engineering, Kinki University; ai@arch.kindai.ac.jp Takayuki Matsushita, Professor, Department of Architecture, Graduate School of Engineering, Kobe University; matusita@kobe-u.ac.jp KEYWORDS: Heating system, Crawl space, Thermal storage, Heat pump, Off-peak electricity. SUMMARY: In the house that has a crawl space with the insulated foundation walls and non-insulated floor over the crawl space, if all the crawl space can be heated, then the entire floor will be heated, and as the result whole the space of the first floor will become the radiant heating environment. We call this heating system Crawl-space heating. In this papar, the crawl-space heating system combined with the thermal storage using an air source heat pump as a heat source is introduced. To verify the efficiency of this heating system, field test was carried out in an experimental house and following facts were revealed. In the period from 23:00 to :00, that is offpeak electrical load period in Japan, the heat pump generates about 80 MJ heat and supplies the heat to the thermal storage equipments. In the same period, about 40 % of the supplied heat was stored in the thermal storage equipments and the balance 60 % was used to heat the crawl space. In the period from :00 to 23:00, that is peak period, the stored heat in the thermal storage equipments was released to the air coming from the fans and was used to heat the crawl space. Under the condition of the crawl-space heating, the radiant heating environment was obtained in the room. In the environment, vertical temperature distribution was small and floor-surface temperature was higher than the space. Furthermore, the difference between the room temperature in the first floor can be minimized as far as the adequate circulation of the warm air in the crawl space is secured and also the thermal conductivity of the floor material is good enough to transmit the heat in the crawl space to the above room. 1. Introduction There are many ways for heating for detached houses. They can be divided into roughly two ways, one is convective air heating and the other is radiant heating. Convective air heating has some disadvantages compared with radiant heating. Convective air heating tends to make the temperature of the lower part of the room lower than that of the upper part. The moving dry air that blows to the body makes the people in the room uncomfortable. On the other hand, radiant heating does not have such disadvantages and it is becoming preferable. As to the heating area of a house, there are roughly two ways. One is partial heating and the other is whole house heating. Partial heating has high risks of causing condensation and vascular disorders such as cerebrovascular disease, whereas whole house heating has low risks of them. To warm up whole house, it is needed to build wellinsulated and highly airtight house and install the suitable heating system. At the house that has a crawl space with the insulated foundation walls and non-insulated floor over the crawl space, if all the crawl space can be heated, then the entire floor will be heated, and as the result whole the space of the first floor will become the radiant heating environment. We call this heating system Crawl-space heating. To make the same heating environment in the first floor using the general floor heating system, it is

needed to install the system in the whole floor of the first floor. On the other hand, in the case when using the crawl-space heating, it is only needed to install the heating system in the crawl space. To heat the crawl space, several heat sources such as an electric-resistance heater or a gas boiler can be considered. If the climatic condition of the area is suitable for using a heat pump (Schibuola 2000), more efficient heating can be expected by using it because it has ability to output several times more thermal energy than the inputted electrical energy. The heat source the coefficient of performance (COP) of which is over 1 is only the heat pump. In Japan, which is comparatively warm climate, the heat pump can be used as a heat source for heating. Furthermore, combining the crawl-space heating with the thermal storage would bring about some benefit in the countries where electrical energy consumption varies greatly during day and night. If some of the peak load could be shifted to the off-peak load period, better power generation management can be achieved. In order to narrow the gap between the peak and the off-peak electrical demand, the electric price in the peak load period is usually set higher than that in the off-peak load period in many countries. Then the shift of electrical consumption from the peak load period to the off-peak load period by using the thermal storage will provide significant economic benefit. In this paper, the crawl-space heating system combined with the thermal storage using the air source heat pump for space heating as the heat source is presented. The schematic diagram of this heating system is shown in Fig. 1. A heat pump, a fun and thermal storage equipment are installed in the crawl space. The heat pump supplies heat to the thermal storage equipment by using cheep off-peak electricity. The thermal storage equipment consists of sensible thermal storage materials and spaces through which air from the heat pump or the fan flows exchanging the heat with the materials. At the off-peak period, some of the heat from the heat pump is stored in the thermal storage materials and the heat not stored is used to heat the crawl space. At the peak period, the stored heat in the thermal storage materials is released to the air coming from the fan and is used to heat the crawl space. FIG. 1: Schematic diagram of crawl-space heating combined with thermal storage using heat pump as heat source. Left: at off-peak load period, Right: at peak load period. There have been some studies of floor heating system combined with thermal storage (For example, Lin et al. 2005). However, heat pump has not been used as the heat source in those studies. There have been some studies of heat pump combined with thermal storage. Badescu (2003) used the thermal storage to store the collected solar energy that was used to operate a solar assisted heat pump system. Long and Zhu (2008) used the thermal storage in a heat pump water heater. Fujita (200) developed a model to study the thermal performance of the crawl-space heating combined with thermal storage using a haet pump as a heat source. However, experimental study about the heating system has not appeared so far. The objective of this work is to verify the efficiency of this heating system experimentally. Specifically, the indoor thermal environments such as vertical and horizontal temperature distributions were measured and the quantities of the stored and the released heat in the thermal storage equipment were estimated by the field tests that were carried out in an experimental house in Osaka, Japan.

2. Experimental Set-up Field tests were carried out at the west end of an experimental house in Osaka, Japan from December 18 to 28, 2006. Figure 2 and Figure 3 show the foundation plan and the first-floor plan together with the temperature measurement points. Figure 4 shows a cross section. FIG. 2: Foundation plan showing measurement points. FIG. 3: First floor plan showing measurement points. FIG. 4: Cross section.

The house was two-storied with other rooms to the east of the experimental section separated by a 1 m wide corridor running from north to south. The wall of the section facing the corridor had the same specifications as the outer walls. The crawl space of the corridor was connected with outdoor through openings of the foundation walls and the corridor was not heated. Therefore, the experimental section could be considered thermally as a detached house. The floor of the experimental section consisted of sleepers, joists, plywood 12 mm thick, and wooden flooring 12 mm thick. The ground surface of the crawl space was covered with concrete 200 mm thick. The foundation walls of the crawl space were insulated by extruded polystyrene 100 mm thick. The edge parts of the slab concrete were insulated by extruded polystyrene of 50 mm thick and 500 mm width. The heat loss from the experimental section to the outdoor per floor area was estimated 2.3 W/m 2 K. A 6.0 kw heat pump was installed in the Northwest section of the crawl space. Two fans were installed also in the Northwest section of the crawl space. Two thermal storage equipments were installed in the west section of the crawl space and they were connected to the heat pump and also to each fan respectively by insulated ducts. Figure 5 and Figure 6 show a horizontal cross section of the thermal storage equipment and a vertical cross section of that. The thermal storage equipments consist of sensible thermal storage materials and spaces through which air from the heat pump or the fan flows exchanging the heat with the materials. The physical properties of the materials are shown in Table 1. The heat pump was set to 30 C and operated from 23:00 to :00 that is off-peak period in Japan. The fans to promote the release of the stored heat were operated from :00 to 23:00. Three auxiliary fans producing airflow 650 m 3 /h respectively were operated to control the air current in the crawl space. As shown in Fig. 2, they were installed to move the air from the West to the Southwest, from the West to the East and from the Southwest to the Southeast respectively. d u ct (d ia m eter1 5 0 ) fro m h ea t p u m p m ea su rem en t p o in t o f tem p era tu re m ea su rem en t p o in t o f a irflo w velo city 2 2 8 X fro m fa n X 2 2 8 4 4 d u ct (d ia m eter1 5 0 ) th erm a lin su la tio n 0 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 2 2 8 3 1,9 2 0 FIG. 5: Horizontal cross section of thermal storage equipment. (unit; mm) 1 0 1 0 1 0 1 0 3 6 5 0 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 1 9 2 3 1,9 2 0 FIG. 6: Vertical cross section of thermal storage equipment. (unit; mm) TABLE. 1: Physical properties of sensible thermal storage materials. Specific heat 954 J/kgK Density 3,50 kg/m 3 Thermal conductivity 2. W/mK

3. Results and discussions Figure shows the vertical temperature distributions at each point shown in Fig. 3 at 12/21 14:00. The temperatures at 50 mm and 1,200 mm above the floor were almost the same and the floor-surface temperatures were higher than the space temperatures. From these results, it was verified that the radiant heating environments were formed in the room by the crawl-space heating. FIG. : Vertical temperature distributions at each point shown in Fig. 3. Figure 8 shows the temperatures at 1,200 mm above the floor in the room on the respective points shown in Fig. 3. The differences between the temperatures of each measuring points were less than 2 C except for the East and the Northeast. Figure 9 shows the temperatures of the respective points of the floor-surface. The temperatures were higher than 18 C except for the East and the Northeast. The low temperatures at the East were due to the low thermal conductivity of the floor material, i.e. Japanese mat called tatami, the thickness of which was 50 mm and its thermal conductivity was about 0.1 W/mK. Figure 10 shows the crawl space temperatures at 300 mm below the floor under-surface in the crawl space on the respective points shown in Fig. 2. The temperatures of the Northeast were lower than the other points. Following reasons are considered. There was no auxiliary fan at the opening of the partitioning foundation wall between the East section and the Northeast section and there was no adequate opening in the partitioning foundation wall between the Northeast section and the Northwest section. Consequently, the warm air from the thermal storage equipment could not circulate to the heat pump or to the fans through the Northeast section. FIG. 8: Temperatures at 1,200 mm above floor in room on respective points shown in Fig. 3.

FIG. 9: Floor-surface temperatures on respective points shown in Fig. 3. FIG. 10: Crawl space temperatures at 300 mm below floor under-surface in crawl space on respective points shown in Fig. 2. These results prove that the difference between the room temperature in the first floor can be small by applying the crawl-space heating as far as the adequate circulation of the warm air in the crawl space is secured. Furthermore, the thermal conductivity of the floor material must be good enough to transmit the heat in the crawl space to the above room. Table 2 shows the quantities of generated heat by the heat pump and those of stored and released heat in the thermal storage equipments. The quantities of generated heat were estimated from the difference between the entering air temperature to the heat pump and the leaving air temperature from the heat pump with the specific heat of the air and the flowing air volume using the Equation (1). HP HP { acpva ( Tout, t Tin, t ) t} Q = ρ (1) g where y23 Q g : quantity of generated heat by heat pump from 23:00 to :00 (J) ρ a : density of air (kg/m 3 ) C V T a P HP out t HP in t : specific heat at constant pressure (J/kgK) : flowing air volume through heat pump and thermal storage equipments (m 3 /s), : leaving air temperature from heat pump (K) T, : entering air temperature to heat pump (K) t and y23 : measurement interval (s), (=300s) means sum from 23:00 of previous day to :00

The quantities of stored and released heat were estimated from the difference between the entering air temperature to the thermal storage equipments and the leaving air temperature from the equipments with the specific heat of the air and the flowing air volumes using the Equations (2) and (3). TS TS { acpva ( Tin, t Tout, t ) t} Q = ρ (2) S y23 23 TS TS { ac PVa ( Tout, t Tin, t ) t} Q = ρ (3) R where 23 and Q s TS in t : quantity of stored heat in thermal storage equipments (J) T, : entering air temperature to thermal storage equipments (K) T TS out t Q R, : leaving air temperature from thermal storage equipments (K) : quantity of released heat in thermal storage equipment (J) means sum from :00 to 23:00 From 23:00 to :00, the quantity of generated heat was about 80 MJ and that of stored heat was about 30 MJ, and the balance 50 MJ was used to heat the crawl space. From :00 to 23:00, the heat pump was not in operation, and the stored heat of 30 MJ was released to the air coming from the fan in order to heat the crawl space. Figure 11 shows the quantities of released heat at each hour from 12/24 :00 to 22:00. From these results, it was verified that about 40 % of the generated heat was stored in the thermal storage equipments from 23:00 to :00 and this stored heat could be used to heat the crawl space from :00 to 23:00. TABLE. 2: Quantities of generated heat by heat pump and those of stored and released heat in thermal storage equipments. Period Generated heat (MJ) Stored heat (MJ) Released heat (MJ) 12/19 23:00 12/20 :00 31 12/20 :00 12/20 23:00 28 12/20 23:00 12/21 :00 8 31 12/21 :00 12/21 23:00 30 12/21 23:00 12/22 :00 6 30 12/22 :00 12/22 23:00 30 12/22 23:00 12/23 :00 6 30 12/23 :00 12/23 23:00 29 12/23 23:00 12/24 :00 31 12/24 :00 12/24 23:00 30 12/ 24 23:00 12/25 :00 9 32 12/25 :00 12/25 23:00 30 12/25 23:00 12/26 :00 31 12/26 :00 12/26 23:00 29 12/26 23:00 12/2 :00 0 12/2 :00 12/2 23:00 28

FIG. 11: Quantities of released heat at each hour from 12/24 :00 to 22:00. 4. Summary and Conclusions In the house that has a crawl space with the insulated foundation walls and non-insulated floor over the crawl space, if all the crawl space can be heated, then the entire floor will be heated, and as the result whole the space of the first floor will become the radiant heating environment. We call this heating system Crawl-space heating. A crawl-space heating system combined with the thermal storage using the air source heat pump as the heat source was introduced. To verify the efficiency of this heating system, field test was carried out in an experimental house in Osaka, Japan. A heat pump, two funs and two thermal storage equipments were installed in the crawl space. In the period from 23:00 to :00, that is off-peak electrical load period in Japan, the heat pump generates about 80 MJ heat and supplies the heat to the thermal storage equipments. In the same period, about 40 % of the supplied heat was stored in the thermal storage equipments and the balance 60 % was used to heat the crawl space. In the period from :00 to 23:00, that is peak period, the stored heat in the thermal storage equipments was released to the air coming from the fans and was used to heat the crawl space. Under the condition of the crawl-space heating, the radiant heating environment was obtained in the first floor. In the environment, vertical temperature distribution was small and floor-surface temperature was higher than the space. Furthermore, the difference between the room temperature in the first floor can be minimized as far as the adequate circulation of the warm air in the crawl space is secured and also the thermal conductivity of the floor material is good enough to transmit the heat in the crawl space to the above room. 5. References Badescu V. (2003). Model of a thermal energy storage device integrated into a solar assisted heat pump system for space heating, Energy conversion and management, Vol 44, 1589-1604. Fujita K., Iwamae A. and Matsushita T. (200). A study on thermal performance of heat storage system connected with heat pump for residential houses, Proceedings of Building Simulation 200, Tsinghua University, Beijing, China, 321-328 Lin K., Zhang Y., Xu X., Di H., Yang R. and Qin P. (2005). Experimental study of under-floor electric heating system with shape-stabilized PCM plates, Energy and Buildings, Vol 3, 215-220. Long J. and Zhu D. (2008). Numerical and experimental study on heat pump water heater with PCM for thermal storage, Energy and Buildings, Vol 40, 666-62. Schibuola L. (2000). Heat pump seasonal performance evaluation: a proposal for a European standard, Applied thermal engineering, Vol 20, 38-398.