Case of Energy System in Northwest China

Abstract

In rural areas of northern China, cooking, household appliances, and winter heating make up the basic energy consumption which almost totally depends on coal until now while renewable energy, such as solar energy, biomass, and wind energy, is abundant in these areas. Great efforts have been made to meet one of basic energy demands with renewable energy. However, unsteady renewable energy has significantly challenged the reliability of the renewable energy system and users’ expectations. Hence, in order to meet multilevel energy demands above with solar energy and biomass and to efficiently overcome the influences of seasons, climates, environment temperature, and other factors on renewable energy production, an energy system of combined heating, power and biogas (CHPB), was developed and tested in a 117.07 m² insulated rural building. The CHPB system is composed of solar water collectors, PV arrays with batteries, thermostatic biogas digesters, and other devices. Besides, to keep the temperature of the biogas digester stable, the solar water collector is also used to heat the building in winter. PV arrays with batteries supply electricity for the system and household appliances, while biogas is used for cooking. The CHPB system shows favorable performances in heating period of 2014–2015, according to the theoretical analysis on efficiency, conservation, and environmental benefits of the system in winter.

The energy supply performance of CHPB in winter is studied experimentally. The test results show that: (1) During the heating periods, the energy supplied by the system used for building heating meets 69% of the building needs. When the daily average ambient temperature is higher than 3 °C, the system is capable of meeting the energy demands of building heating completely, while when the accumulated daily solar radiation is less than 14 MJ/m², the system fails to meet the energy demands of building heating; (2) The total biogas produced by the system in the test is 110.71m³, with an average methane content of 54.74% which always meets the cooking fuel demands of the residents; (3) Most of the time, the daily electricity generation is higher than the electricity consumed by CHPB and relies on batteries. Besides, the generated electricity had met the demands of CHPB all the time, and the system can meet the electricity demands of the residents partly in the heating periods and completely in the nonheating periods.

Nomenclature

Q _H

the total heat consumption of buildings (W)

Q _wh

the basic heat consumption retaining structure (W)

Q _lfs

cold air penetration heat consumption (W)

Q _lfq

cold air intrusion heat consumption (W)

Q _m

total heat requirement of biogas digesters (W)

Q _W

average heat consumption of domestic hot water (W)

Q ₀

building energy output (W)

Q ₁

the amount of heat required to heat the feedstock (W)

Q ₂

heat loss of biogas digesters’ heat transfer pipelines (W)

Q ₃

heat consumption of biogas digester’s exterior-protected construction (W)

Q ₄

biological heat produced by anaerobic reactions (W)

${Q}_{\mathrm{i}}^{\prime }$

revised the basic heat transfer of the structure (W)

Q _i

basic heat transfer of the enclosure structure (W)

ε

total correction(the total correction rate, the risk attaching rate and the height addition rate)

t _i

the indoor air calculates the temperature (°C)

q_r

the quota of hot water (L/(person • d))

ρ _w

hot water density (kg/L)

t _f

design the cold water temperature (°C)

λ ₁

the coefficient of the polystyrene’s thermal conductivity (W/(m²·K))

λ ₂

the coefficient of the soil’s thermal conductivity (W/(m²·K))

λ ₃

thermal conductivity of concrete slab (W/(m· K))

λ ₄

thermal conductivity of polystyrene board (W/(m· K))

h ₁

internal convection heat transfer coefficient of digesters (W/(m²·K))

h ₂

heat transfer coefficient between the outer surface of insulation layer and environment (W/(m²·K))

δ ₃

the thickness of concrete slab (m)

δ ₄

the thickness of biogas insulation layer (m)

m_L

the daily volume of fresh feed into the digester (kg/day)

T _S

the temperature of the fresh feed (°C)

T _A

the temperature of the media outside the tank (°C)

d _i

the internal diameter of the heat transfer pipe (m)

a

buried depth (m)

V

the interior volume of the room (m³)

N

the number of ventilation (times/h)

ρ

air density

c_P

cold air constant pressure specific heat capacity (kJ/(kg ·°C))

F _i

the area of the envelope (m²)

K _i

the heat transfer coefficient of each envelope (W/(m²·K))

t_e

the calculation of outdoor heating temperature (°C)

m_w

the number of water users (people)

c_W

specific heat capacity of water (J/(kg·°C))

t _r

design the hot water temperature (°C)

c_L

specific heat capacity of liquid (kJ/(kg·°C))

T _D

temperature of liquid in biogas fermentation (°C)

L

the length of the root pipe (m)

β

local heat loss coefficient

d ₀

the outer diameter of the heat-insulating pipe (m)

F

the external surface area of biogas digesters (m²)