Dural-roof greenhouse is a new form of solar greenhouse (Figure 1), which was designed for the purpose of reducing heat loss and improving land use efficiency. The newly designed dual-roof greenhouse locates in Fangshan District, Beijing (latitude 39˚40'N). The floor size of south room: length × width = 156 m × 8 m ( 1248 m ² ); north room: length × width = 156 m × 5 m ( 780 m ² ).

About 26 circulating fans (totally 13 groups, 13 fans at the height of 1.8 m and 13 fans at the height of 0.5 m above ground, and two fans are one group) were installed in walls for exchanging air between south and north rooms. The position of vents and system was shown in Figure 2. Dual-roof solar greenhouse will transfer the extra heat from south room to north wall and north room for storing during day time when south room is warmed with the “greenhouse effect”.

In the night, the heat stored in north wall and north room will, reversely, reheat the south room where grows thermophilic vegetables such as tomato and cucumber. Meanwhile, the north room could also be utilized to grow vegetables such as mushroom preferring lower lights and temperature environments. The heat exchanged between north and south rooms at each group fan [5] during day time were quantified with Equation (1).

where, E_fc,k,τ is heat exchanging rate of the fan group k during time τ, kJ; V_fc,k,τ is volume of air flowed through the fan group k during time τ, m³; ρ_{air, τ} is air density during time τ, kg/m³; h_{fc-in,k, τ} and h_fc-out,k,τ are mean air spe- cific enthalpy at the inlet (air from north room) and outlet (air from south room) of fan unit k, respectively, during time τ, kJ/ kg.

The air specific enthalpy at each fan unit [5] was calculated with Equations (2) and (3).

where, d_fc-in,k,τ and d_{fc-out, k,τ} are mean air humidity ratio at the inlet (air from north room) and outlet (air from south room) of the fan group k, respectively, during time τ, kg (water)/kg(dry air); t_fc-in,k,τ and t_fc-out,k,τ are mean air temperatures at the inlet and outlet of the fan unit k, respectively, during time τ, ˚C.

A monitoring study (Figure 3) was conducted in winter of 2012 -2013 in the newly designed and constructed dual-roof solar greenhouse in order to assess the heat preserving capability. The south-room was grown with tomato, and the north room was grown with mushroom during the test. The indoor temperature, relative humidity, CO₂ concentrations, solar radiation indoor and outsides were monitored and recorded with an integrated data-logger (Beijing PAIDE Science and Technology Development Co., Ltd., Beijing). Five monitoring locations (in the middle of the room in south-north direction, at height of 1.5 m above the floor) were chosen for south and north rooms, respectively. The time step of data recording was 30-min.

The indoor air temperatures of south and north rooms collected in December of 2012 were shown in Figure 4.

From Dec.1st to 5th, averaged temperature in south, north rooms and outdoor were 12.1˚C, 0.6˚C, and −2.9˚C, respectively. South room temperature is satisfying the basic requirement for leaf vegetables growing. There was 15˚C difference between south room and outdoor. Besides, the highest T in south room can be reached at 30˚C at noon with the increasing of solar radiation. The exchanged CO₂ between north and south rooms and its influence on plants growing will be studied in the near future.

It is reported that the heat load of solar greenhouse is about 0.065 to 0.075 kW/m ² in Beijing area and heating

period is 146 d [6] , which means the energy preserved by dual-roof solar greenhouse is about 87 kW or 7547 MJ/d (floor area of south room is 1248 m ² ). Total energy saved by dual-roof in whole winter was about 1101994MJ (about 1.1 × 10⁷ MJ). As the heat value of standard coal is 29,306 MJ/t, about 37.4 t coal was saved for heating the greenhouse.

Traditional designing of solar greenhouse has to consider shade from frontier greenhouse, therefore, the area between greenhouses usually cannot be utilized [12] (Ma et al., 2006). However, the dual-roof solar greenhouse designed in this study make used of the land area fully by building the north room. Considering the ratio of north room and south, the land use efficiency was improved by 62.5%, which is very important for the fast growing metropolitan city like Beijing.