Author : Ir. Hui Wing On     China Team Engineering Consultants Limited

Energy consumption and environmental protection of air conditioning systems have always been people's concerns. Coastal cities in PRC possess of rich sea water resources, seawater temperature is considered stable, which is higher than outdoor air temperature in winter and lower in summer season. Therefore, when seawater can be used as the heat source of central air conditioning systems, should be a natural idea. This paper mainly studies the feasibility of using shallow surface seawater as the cooling/heating source for a central air conditioning system in one of China Team’s high-end resort hotel projects, and analyzes the technical difficulties and recommended solutions.

PROJECT OVERVIEW

The second-phase development of a high-end resort hotel by the waterfront of Phoenix Bay in Xiangzhou District, Zhuhai City. This phase development has a total built area of 69,000 square meters and a total of 115 sea-view luxurious guest rooms. 24 hr. Air conditioning, heating, and domestic hot water are required. A 4-pipes system will be used for air conditioning and there is a demand for domestic hot water throughout the year. This project is right next to the beach separated by a parapet wall. The project site is about 560 meters long from east to west. The geographical location of the project is precious and picturesque; the owner is willing to invest and use seawater source heat pump system as the cooling / heating source for the year round operation in order to maintain a beautiful and unique scenery environment without any pollution to the atmosphere.

Phase I of the High-end resort hotel in operate.

Existing waterfront condition of the hotel phase 2 project site.

ZHUHAI CLIMATE

Zhuhai City is located on the west bank of the Pearl River Estuary and borders the vast South China Sea. It is a typical south subtropical monsoon maritime climate. The air temperature is high throughout the year, with an average annual temperature of 22.5 ℃; an average annual relative humidity of 81.6%. Relative humidity from March to June is above 84%, winter season is relatively short, the four seasons are:

Spring : from March to May,

Summer : from June to August,

Autumn : from September to November,

Winter : from December to February.

The annual average temperature is 22.5 ℃, and the inter-annual temperature change is generally between 21.6 ℃ ~ 23.3 ℃. The highest annual average temperature recorded is 23.3 ℃; the lowest is 21.6 ℃. The daily extreme maximum temperature is 38.5 ℃, and the daily extreme minimum temperature is 2.8 ℃. The coldest period is from December to February of each year. Among them, January is the coldest month, and the average monthly temperature in January is 15.0 ℃. July is the hottest month and the average monthly temperature is 28.5 °C; air temperature gradually cools down from October.

Tidal nature of the tidal mouth of the Tamsui is a case of irregular semidiurnal tide mixed type, a day in the sun, tidal two up two down, ranging from day now as obvious.

PROJECT RESEARCH OBJECTIVES   

With reference to several successful cases of using seawater as a cooling/heating source for air conditioning in South China region, developer of this project is willing to contribute to environmental protection, and not cause any pollution to the atmosphere because of the development of this project. The goal is to study the use of shallow seawater as air conditioning heat source. Feasibility study on the production of domestic hot water by combining cooling/heating sources with heat pump units was carried out at the same time. The calculated peak cooling capacity of the phase of this hotel is 8,308 KW, the peak heating capacity in winter is 2,076 Kw, and the peak domestic hot water heating capacity in winter is 4,449 Kw. Considering the climatic conditions in Zhuhai area, plus this project has very rich shallow low-grade heat energy, referring to the energy saving control indicators of the same type of projects in Guangdong Province, the annual new energy replacement rate of this project can reach 100%, including seawater source heat pump The average annual comprehensive energy efficiency of heat is relatively high, and the COP value of cooling conditions is about 3.8-4.8, while the comprehensive COP value of heat recovery (heating + cooling) is about 5.5-6.6. Therefore, seawater source is the main consideration of this study.

Resources in the waters near Zhuhai

Zhuhai City is located in the Pearl River Estuary area. The tides in Zhuhai ’s sea area are mainly caused by the influence of the terrain, rivers and streams, and meteorological factors after the Pacific tide waves passed through the Bashi Strait and the Bahrain Strait. They are    irregular half-day tides and have unequal tide days. The annual average tidal range of each station in the city is about 1 meter, which belongs to the weak tide estuary.

Due to the influence of river topography and tidal waves, the rising tide duration of the tides in the sea area is not equal:

Near the Pearl River estuary, the average high tide lasted about 5 hours and 30 minutes, and the average low tide lasted about 7 hours .

Tide gap occurs due to astronomical factors and friction, that is, the time difference between mid-day and high tide. The interval between high tides is 7 hours and 30 minutes to 9 hours and 30 minutes, while it is around 10 hours near the coast.

Due to the influence of the Pacific typhoon and the South China Sea typhoon, the coastal waters increased. According to statistics, from 1848 to 1949, the Zhuhai area suffered 60 typhoon surge disasters, and the storm water level was mostly above 2 meters, with a maximum of 3.37 meters.

Salinity is restricted by factors such as runoff and tidal current, so there is an obvious law of temporal and spatial changes. Spring, summer, autumn, winter:

The surface salinity is <10, <1, <1, <1;

The salinity of the bottom layer is 9, 4, 1, and 6 higher than the surface layer, respectively .

DESIGN PRINCIPLES

The beach next to the project is an artificial beach. It is not possible to use deep-water port mode to direct extract seawater as a cooling/heating source by set up a seawater pumping house. Deep well method is proposed, three test wells in the site to extracted groundwater for water quality testing. According to the terrain and road status during field testing, in order not to damage the on-site environment and facilitate construction, the positions of wells 1 # and 2 # were determined first, and then based on the test result of two wells to fix the location of the well3 #. Specialized professional company where responsible for drilling, water sampling collection testing.

In order to ensure the validity and accuracy of the testing data, the drilling equipment and drilling specifications used in the testing wells are exactly the same as the wells when actually implemented in the project. The upper (20m ) borehole dia. of the test well is 300 mm, the lower borehole dia. is 140 mm, and the total drilling depth is 150 m.

TEST TARGET

Conduct a survey on the condition of the project site and conduct a detailed survey of shallow geothermal energy resources. "The objective of this test was to identify the location and temperature of marine water resources and groundwater resources within the available range of the project (rock formations and near-sea water), Water quality, resource quality, rock stratum structure, thermophysical parameters and distribution pattern, aiming to find out the most suitable way of using shallow geothermal energy of this project to ensure efficient and stable energy-saving operation of this project's energy system. Specific test direction are:

Geological survey: to find out the distribution pattern geothermal field, lithology structure, hydrogeological conditions and thermophysical parameters of rock and soil layers;

Suitability zoning: On the basis of geological survey, according to different utilization methods (such as seawater heat exchange well type, water intake heat exchange type, compound type, etc.), multiple analysis methods are used to perform suitability zoning. The main considerations of suitability zoning were: seawater temperature, ground temperature, loose layer thickness, water inflow, cumulative thickness of aquifer, seabed depth, tide, ocean current, air temperature, sunshine, etc .;

Assessment of available resource potential: The purpose of the assessment of available resource potential was to evaluate the recoverable amount of shallow geothermal energy and storage of geothermal energy by evaluating the resources of shallow geothermal energy (including seawater energy) within the red line (site   boundary）of the project, water quantity to ensure it meets the long-term sustainable and stable operation of the system;

Environmental impact assessment and monitoring: The impact of the development and utilization of shallow geothermal energy resources on the surrounding environment is related to the important factors of the sustainable development of shallow geothermal energy resources, so the surrounding environment was also monitored and evaluated simultaneously.

The content of environmental impact assessment was to evaluate the heat balance in groundwater (or seawater) and strata, changes in shallow ground temperature field and the possible impact on the ecological environment according to different heat exchange methods of the heat pump system.

The evaluation is divided into two aspects:

Evaluation of the impact on the geological environment, in which the thermal well type heat exchange system mainly evaluates the influence of the thermal conductivity of the rock formation and the heat release of the circulating water on the underground rock formation environment; on the development of the surface water heat exchange system Impact, whether the effect of backwater on the seawater environment will cause the destruction of the ecological environment of aquatic organisms in surface water;

Shallow ground temperature recoverability and shallow ground temperature field change trend evaluation, through heat pump simulation of heat pump system, evaluate shallow ground temperature recoverability, estimate the sustainable use of shallow ground temperature energy, and evaluate the changes of ground temperature field Effects of microorganisms in the formation and seawater.

On-site drilling in September 2019

TEST CONTENT

Implementation plan of drilling and protection technique at the site;

The fissure situation of the deep underground rock layer on the shore, the source, quantity and salinity of the fissure water that accumulates;

Permeability testing of shore formations;

The temperature and comprehensive thermal conductivity of the deep underground rock layer on the shore under different working conditions;

Seawater depth, slope and underwater temperature of the shoals on the shore;

Water quality analysis, including temperature, salinity, microorganisms, sediment and other parameters.

The installation of the heat-exchange well wall tube and the implementability of the pressure grouting backfill scheme;

Determination of dynamic water level of groundwater;

Water injection test and pumping test;

Drilling geological samples

TEST WELL RECORD

WATER INJECTION AND PUMPING TEST

(1) Water injection test:

Drilling requirements: stop drilling in the test section of water injection test;

Groundwater level observation: Before water injection test, the groundwater level observation was carried out, the water level observation interval was 5min, and the water level observation was ended when the data amplitude of 2 consecutive observations was less than 10cm, and the last observation value was used as the groundwater level calculation value;

Water stop in the test section: The water stop method by using water shut-off with clay end to ensure reliable water stop;

After the test section is isolated, inject fresh water into the casing so that water level in the casing is higher than the groundwater level by a certain height and remain fixed. Use a flow meter or measuring barrel to measure the injection flow;

Measurement method: Start to measure every 5 minutes and measure 5 times in succession; thereafter measure every 20 minutes and measure at least 6 times continuously. When the difference between the injection flow rate measured in two consecutive times is not more than 10% of the last injection flow rate, the test is completed and the last injection flow rate was taken as the calculated value;

When the water leakage in the test section is greater than the water supply capacity, the maximum water supply was recorded.

(2) Pumping test:

Adopt a steady flow hole pumping test, three drawdown was carried out, pumped to a pressure measurement in the bore of the tube was measured, drawdown prevail each time equal to the difference in depth between the drop, from small to large drawdown, the minimum drawdown should not less than 0.5m;

After the hole was washed, the test pumping was carried out, and its depth gradually increased, and the duration after reaching the maximum depth was not less than 2h. During the pumping test, observe the changes in the water output and the water level of the pumping hole to determine the maximum drawdown of the steady flow pumping;

Before the formal pumping, the static water level observation was carried out in a 30 mins. interval, it was considered stable when the variation within 2h was not more than 2cm, with no continuous upward or downward trend;

At the 5th, 10min, 15min, 20min, 30min, 40min, 50min, 60min after the start of the pumping, the dynamic water level and the water output were observed once, and every 30min thereafter;

Dynamic water level stability standard: ground centrifugal pumps and submersible pumps were used for pumping, and the water level fluctuation of the pumping hole should not more than 3cm; when air compressor is used for pumping, the water level fluctuation of the pumping hole should not more than 10cm;

A stable water pumped in the standard duration of stability: 100%. 5% Q - Q <average maximum and minimum amount of water ' Q , and continues to increase the amount of water or no tendency becomes smaller;

The stability duration is not less than 4 hours;

Immediately after the test is stopped, the recovery water level is observed. After the pumping is stopped, the water level is observed once at 1min, 2min, 3min, 4min, 6min, 8min, 10min, 15min, 20min, 25min, 30min, 40min, 50min, 60min, 80min, 100min, 120min , Observing every 30 minutes thereafter;

After the test, the depth of the hole was measured, re-measure the height of the hole (tube), and check the precipitation in the hole.

FIELD TEST RESULTS

SEAWATER SOURCE COMPOUND HEAT PUMP SYSTEM ADOPTING COASTAL HEAT WELL TECHNOLOGY

The fissure water of the rock layer in the coastal thermal well comes from the seawater of the seabed fissure and the rock fissure of the land to form brackish water, which enters the coastal well after heat exchange with the granite formation. Coastal heat wells need to be located as close as possible to the sea to ensure that seawater is the direct supply source of well water. After the seawater exchanges heat with the rock layer through the fissures, the temperature decreases in summer or increases by about 5℃ in winter, which can improve the efficiency of the heat pump unit. According to the results of on-site testing, the water output of the underground fissures is not enough to meet the entire heat dissipation of this hotel phase, so this heat well system needs to be designed as a closed system, using the accumulated underground fissure water pumped out to form a closed cycle, in which heat will be discharged to the granite, part of the heat will be taken away by the cracked brine, and the rest of the heat will be digested by the rock layer, which will be taken out as heat source in winter operation,   thereby forming a seawater source compound heat pump system relying on the granite rock formation.

Design coast heat well with direct heat exchange system is better than surface sea water, summer temperatures decreased in every 5℃, the heat pump unit efficiency will improved by 15%, and the well water filtered through rock possess a better quality than surface sea water directly pump from surface, a better cost control of the project to ensures stability of later operations.

SEAWATER SOURCE HEAT PUMP UTILIZATION SCHEME

This solution applies air-conditioning heat recovery technology, heat recovery heat pump unit heat up domestic hot water and provides air-conditioning chilled water water simultaneously. The integrated COP is
higher and at a low running cost. This is the characteristics of water source heat recovery heat pump which provides hot water and supply air conditioning cooling at the same time In air conditioning season, domestic hot water will be provided as a byproduct of the heat recovery heat pumps according to the priority of actual running capacity required of central air-conditioning system. Should the heat pumps be started to top up heat required for the domestic hot water when heat recovery heat pumps are not producing enough heat. In non-air-conditioning season, domestic hot water is directly produced by water source heat pump units.

Summer working conditions (air conditioning heat recovery): The demand for domestic hot water in summer is relatively small, and the heating required from air conditioning heat recovery will correspondingly reduced to ensure the long-term efficient operation of air conditioning system.
In the case when excess heat is recovered, this is considered as cooling only mode, condenser of heat recovery heat pumps will be cool down by seawater to ensure the cooling effect and increase the COP value of this cooling mode.

Winter conditions: when there is no air-conditioning cooling required, water source heat pump will be the main heating equipment to produce domestic hot water, the well water enters the evaporator, and adopts the single heating operation of the heat pump unit to ensure domestic hot water supply and also improve the COP value of heat pumps.

COOLING & HEATING BALANCE ANALYSIS AND CALCULATION

Theoretical background of cold and heat balance

(1)     The ground source heat pump increases the low-level heat energy in the ground through heat pumps in winter to heat up the hotel, and at the same time reduces the temperature in the ground, that is considered as cold storage for summer use; in summer, heat in this hotel transferred to the underground and store heat in the ground for winter use.
This feature determines that the technology is suitable for areas where summers is hot and   winter is cold, loads on cold and heat in winter and summer are comparable.
If the system is operated for a long time under an unbalanced condition, soil temperature will gradually rise or fall, resulting in deterioration of the heat exchange environment and a decline in heat exchange efficiency, which will affect the efficiency of the heat pump equipment and operation cost will become higher and higher. During the operation of the ground source heat pump system, depending on the load characteristics of the building, the following three situations will occur:

The cooling and heating load of the building is basically balanced or unbalanced, if unbalance rate is small, system releases heat ≌ absorbs heat;

The cooling load of the building is greater than the heating load, at a large unbalance rate, the heat release of the system> heat absorbs;

The heating load of the building is greater than the cooling load, at a large unbalance rate, heat release of the system <heat absorbs.

According to the actual measurement and theoretical calculation of this project, the unbalance rate of 50% is used as the limit, that is, unbalance rate is small (within 50%), due to high-quality thermal diffusion capacity and heat storage capacity of the underground granite formation, this heat unbalance affects the heat pumps operation is still considered acceptable.

(2) Thermal balance calculation of this project

The estimated summer cooling load for this hotel phase is 8,308kw;

Winter hot water load and heating load is 14,238kw / 6 + 2,076kw = 4,449kw;

The heat absorption in winter season is 53% of that in summer.
The ground source heat pump unit releases cooling capacity to the underground rock in winter. During summer conditions, it is necessary to turn on the ground source heat pump unit for cooling (and heat recovery for hot water) to release the same unit of heat to the rock formation to achieve the annual heat balance of the underground.
Taking full advantage of the characteristics of the granite strata with fast heat dissipation and the linear distribution of coastal thermal wells, underpin the recovery of the underground temperature field.

Calculation of the number of coastal thermal wells

The number of coastal hot wells for this phase of the hotel is calculated based on the maximum cooling load demand in summer 8,308kw;

Total heat removal (Q) of the coastal thermal well system in summer:

Q = 8,308kw
×
1.25 = 10,385kw;

The maximum heat recovery of the heat pump unit in summer is 2,058kw, and the actual heat removal is 8,327kw;

According to the thermal physical property test results of coastal thermal wells, the heat transfer index (a) per unit metre: 250w / m, and the heat exchange capacity of a single well is 50kw / well

Number of coastal thermal wells needed:

Number of wells: 8,327kw / 50kw / well = 166 wells;

With 2 additional observation wells, the total number of wells tentatively designed as 168.

Coastal well location

Coastal hot wells should be arranged as close to the coastline as possible, arranged at a distance of 6-8 meters (well to well distance), and distributed linearly on both sides of the internal drive way.

TECHNICAL REQUIREMENTS OF SEAWATER HEAT PUMP FOR THIS PROJECT

The key to the heat pump unit's long-term and efficient operation is its heat exchange efficiency.
If brine water directly enters the unit, brine is strong corrosive and the life of conventional equipment will be very short, which affects the heat exchange efficiency of the unit. If titanium alloy unit is used, the investment cost will be high.
Therefore, the indirect heat exchange method is adopted for the heat exchange between the heat pump working medium and the brine water, intermittent heat exchanger is used. Seawater does not directly enter the heat pumps, heat pumps operate through a secondary fresh water circuit effectively and stably.

Basic technical parameters:

Brine water temperature: The average annual water temperature is 24 ° C.

Water temperature of secondary circulation water: 26-31 ℃.

Hot water inlet and outlet water temperature:

circulating heating inlet water temperature is 50 ℃; hot water outlet temperature is 55 ℃.

Local tap water temperature: 13 ℃ in winter and 20 ℃ in summer.

Selection of water source heat pump unit

High Temperature Heat Pump (screw compressor type) having a high water temperature, high environmental resistance characteristics, excellent performance using environmental friendly refrigerant, to maintain a high condensation temperature while cooling, using heat recovery technology, 55 ℃ ensure production
of
domestic hot water. In order to ensure the heating of this hotel phase when the extreme temperature appears in winter, uses a high-efficiency centrifugal heat pump unit, which significantly improves the heating capacity, and can also guarantee the heating demand when the screw machine fails.

Energy station selection table

ENERGY EFFICIENCY ANALYSIS

The coastal heat well seawater source heat pump system has very good energy saving and emission reduction effects:

Obviously, this system operates at a very high efficient, and the heat pump can be adjusted flexibly according to indoor / outdoor temperature;

The running cost is lower than traditional heating and cooling methods, and the operation and maintenance cost is lower than gas boilers;

The benefits of energy saving and emission reduction are effective.
In addition to consuming electrical energy, the coastal heat well seawater source heat pump system does not consume any other non-renewable energy sources, such as coal, gas, and oil. The project can save a large amount of standard coal every year and reduce the emission of sulfur dioxide, nitrogen oxides, particulate matter, carbon dioxide and soot;

The construction plant room area such as the construction of cooling towers site and boiler rooms are eliminated, and smoke pollution and noise interference are effectively avoided.

Compared with conventional energy sources, the comprehensive use of seawater source heat pumps has a significant energy saving effect.

CONCULSION

According to the above energy analysis and technical and economic analysis, the seawater source compound heat pump system of coastal heat well technology used in the energy station of this hotel project meets the requirements of environmental protection and has significant economic benefits. This project uses geothermal wells coast efficient heat transfer technology, from project feasibility studies to implement a complete R&D processes and management processes to ensure the success of the project. The system is expected to have a total investment of 18.91M RMB (not including the construction cost reduction for the reduction of the plant room area) Compared with the traditional system, An annual saving 1.51M RMB on electricity costs is expected i.e. 40% reduction in operating costs. The static return of this investment is about It is 3.23 years. In summary, the use of coastal thermal wells in this project has significant social and environmental effects, is technically and economically feasible, and the investment payback period is reasonably controllable.