How heat pumps work: criteria for heat sources evaluation...criteria for heat sources evaluation Paolo Conti and Walter Grassi University of Pisa and UGI Corresponding author email:

How heat pumps work: criteria for heat sources evaluation Paolo Conti and Walter Grassi University of Pisa and UGI

Corresponding author email: [email protected]

XIV Int. Conference on Science, Arts and Culture. Veli Lošinj, 2014.

mailto:[email protected]

Heat pumps: basic theory

What is an heat pump?

Heat pumps is a device able to

transfer heat from a cold source to an

hot source, against the natural

direction of flow. To do that, driven

energy is required (heat or work)

Coefficient of performance Heating mode

𝐶𝑂𝑃 =𝑄𝑇

𝑄𝑇 − 𝑄𝐹

Cooling mode

𝐸𝐸𝑅 =𝑄𝐹

𝑄𝑇 − 𝑄𝐹

2

Conti & Grassi: How heat pumps work: criteria for heat sources evaluation . XIV Int. Conference on Science, Arts and Culture. Veli Lošinj, 2014.

Heat to

hot source

Driven energy

(Heat or Work)

Heat from

cold source

Gra

die

nt of te

mpera

ture

QT

QF

Heat pumps: basic theory 3

NAME DEFINITION TIPICAL VALUES

COP/EER Useful thermal power divided by

power input 3.5 ÷ 5 / 2.5 ÷ 3.5

<COP> / <EER>

Average COP/EER

Useful thermal energy divided

by total energy input. The

coefficient refers to a specified

time interval.

3 ÷ 4 / 2.5 ÷ 3

SCOP / SEER

Seasonal COP/EER

Useful thermal energy divided

by energy input. The coefficient

refers to the entire

heating/cooling season.

3 ÷ 4 / 2 ÷ 3

PER

Primary Energy Ratio

Useful thermal energy during a

season divided by primary

energy input

1.2 ÷ 1.6 / 0.8 ÷ 1.2

Table 1. Energetic indexes for electrically-driven heat pumps.


Heat pumps: basic theory 4

Thermodynamic reference cycle Components diagram


T –

Tem

pera

ture

[K]

s – Entropy [J/kg]

A B

C

D

Satura

ted liq

uid

Liquid + Vapor

Vapor

Saturation curve

Exp

ansio

n valve

A B

C D Condenser

Com

pre

ssor

or

Ab

sorb

er

Evaporator

Reference cycle vs. real systems

HP efficiency depends on condensing/evaporation temperatures (not sources)

𝑇 𝑆𝐻 < 𝑇 𝑓𝐻

𝑇 𝑆𝐶 < 𝑇 𝑓𝐶

𝑇 𝑆 -> thermal source

𝑇 𝑓 -> operating fluid

𝐶𝑂𝑃 𝑇𝑠 > 𝐶𝑂𝑃 𝑇𝑓

HP efficiency is strongly affected by components performance

5

A B

T - [K

]

s – [J/kg]

C

D

QF

QH

𝑇 𝐻

𝑇 𝐶

𝑇 𝑆𝐻

𝑇 𝑆𝐶


Thermal sources VS. operating fluid

Reference cycle vs. real systems 6

Effects of the variation of evaporation/condensing temperatures

Nominal data refer to standard rating condition of thermal

sources (e.g. UNI EN 14511-2:2013)

Nominal performances

Heating capacity – kW 15.1

Total power input – kW 3.6

COP (only compressor) 4.2

Secondary fluid (Evaporator): Inlet 10°C / Outlet 7°C

Secondary fluid (Condenser): Inlet 30°C / Outlet 35°C


7

0

2

4

6

8

10

12

14

16

-10 -5 0 5 10 15 20 25 30

TfC – [°C]

Thermal capacity - kW

COP

TfH – 35°C

TfH – 35°C

TfH – 45°C

TfH – 45°C

∆COP/∆Tf~0.1 [1/°C]

∆QT/∆Tf~0.25 [kW/°C]



8

𝑷𝒕𝒉,𝒐𝒖𝒕 CR<1



Effects of the capacity ratio

𝐶𝑅 =𝑄 𝑢

𝑄 𝐷𝐶

where:

- 𝑄 𝑢 is the useful thermal power delivered by the HP;

- 𝑄 D𝐶 is the maximum capacity of the HP unit, when

operating at the temperatures of the thermal sources.



0

0,2

0,4

0,6

0,8

1

0 0,2 0,4 0,6 0,8 1

CO

P/C

OP

DC

CR

COP penalization factor depending on control type (UNI EN 14825:2012)

Fixed capacity units

Variable capacity units


Classification and Terminology 11

Air Source Heat pump

systems - ASHPs

Ground Source Heat pump

systems - GSHPs

Ground-coupled HPs - GCHPs

Vertical GCHPs Horizontal

CGHPs

Groundwater HPs - GWHPs

Surface-water HPs - SWHP


12

Evaporator

Condenser

Hot air

INLET Cold air

OUTLET

Air Source HPs


13

Vertical GCHPs


Horizontal GCHPs 14


Groundwater HPs 15


Surface-Water HPs 16


Which source should I use? 17

Ground

Alternative Technologies

Air

Water


Qualitative comparison 18

Suitability Availability Installation

Cost O&M Cost Temperature

ASHPs GOOD EXCELLENT LOW MODERATE VARIABLE

Vertical

GCHPs MODERATE

GOOD /

EXCELLENT HIGH MODERATE GOOD

Horizontal

GCHPs MODERATE

MODERATE/

GOOD MODERATE MODERATE

GOOD /

EXCELLENT

GWHPs GOOD GOOD MODERATE MODERATE/

HIGH

GOOD /

EXCELLENT

SWHPs GOOD MODERATE MODERATE MODERATE/

HIGH GOOD


Real systems layout 19


End-user loop(thermal load)

HPGeothermal loop

Back-upgenerators

Ground Source

BH

Es

Ground Ground Heat

Exchangers (GHEx) Geothermal loop HP equipment

Back-up

generators End user loop

Thermophysical

properties

Number, depth,

diameter,

arrangment

Flow rate and

operating

temperature

Capacity, Type,

Control

Capacity,

efficiency

Puffer and

thermal loads

(power and energy)

GSHP Systems

HVAC Systems

Share of loads delivered with

a SCOP/SEER higher than

alternative technologies

Operating Temperature at

the evaporator/condenser

• Thermal sources

• Heat transfer apparatus

Capacity ratio

• HP sizing and control

• Thermal load pattern

Capacity ratio

𝐶𝑅 ∝𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑎𝑣𝑖𝑛𝑔𝑠

𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐶𝑜𝑠𝑡𝑠

Initial costs

HP capacity + GHEXs

Total energy savings

(HP + Back-up)

Prices/Fares of electricity and natural gas

20

Primary ENERGY savings Economy

What determines a proper work by

GSHPs?


What should be the goal? 21

What is the goal? Maximize benefit-cost ratio (energy/economy) of overall system

(GSHP & Back-up generators)

Evaluate the optimal fraction of building load (energy!) delivered by

GSHPs.

Nominal capacity of GSHP system is a variable and not the final result

What are the main design variables? HP capacity & GHEXs (depth, number, arrangement…)

Management strategy during operation time.

What should be avoided?

Rough & hasty approach to design process

No consideration on the management of system during operation time.


The design process:

a cost-benefit optimization 22

Energetic systems design (especially GSHPs) consists in a

comprehensive procedure aimed at evaluating the

performance of the system during its operational life for cost-

benefit considerations

Sizing process, feasibility study, performance analysis should to

be considered as the very same activity

Further details on this design approach can be found in: “Proposal of a Novel Design Procedure for GSHP Systems”

W. Grassi, P. Conti, D. Testi – EGC2013 proceedings.

“A Design Method for Ground Source Heat Pump Systems Based on Optimal Year-Round Performance –

Part 1: Model Definition and Discussion”, submitted to Applied Energy.


Thermal sources assessment

operative conditions vs. initial conditions 23

Collect all possible information about undisturbed/initial state of the external thermal sources

• Air (ASHPs): annual climate;

• Water (GWHPs and SWHPs): Aquifer depth, temperature, pumping test…

• Ground (GCHPs): temperature, thermophysical properties, TRT…

Evaluate thermal load profile (second thermal sources)

Heat delivery temperature

Power and Energy needs

Does GSHP operation alter the thermal sources?


Air/water vs. ground 24

Ground has a huge thermal capacity, governing its

temperature evolution

Current performance of the GSHP depends on the

full history of heat exchanges (sizing and control

strategy)

Air and water sources are weakly affected by past

operation


Feasibility of GSHP projects:

a case study 25

0,8

0,85

0,9

0,95

1

1,05

1,1

0 0,2 0,4 0,6 0,8 1

En

IN/E

nIN

-AIR

Load share at the ground source

0 (AHP only)

2x100

4x100

6 x 100

10 x 100

15 x 100

20 x 100

30 x 100

BHEs depth


We consider an heating system made of a GCHP and an ASHP (as back-up).

Feasibility of GSHP projects:

a case study 26

Minimum energy consumption corresponds to the optimal synergy between air and ground sources.

Energy savings show a saturation trend at elevate BHEs number, hinting an unfavorable benefit-cost ratio for oversized systems.

BHEs number and depth have to be chosen seeking the optimal tradeoff among installation costs and corresponding achievable savings.


Feasibility of GSHP projects 27

Feasibility and Sustainability

Equipment performances

Sizing and control

strategy

Thermal sources characteristics

Thermal load profile


Seawater as thermal source

Preliminary considerations

Seawater temperature is

less variable than air

one

Seawater temperature is

weakly affected by HPs

operation

Maintenance required

Feasibility study

required (always!)

28

12°E 13°E 14°E 15°E 16°E 17°E 18°E 19°E39° N

40° N

41° N

42° N

43° N

44° N

45° N

46° N

Ancona

Vieste

Split

1000

200

MAR IONIO

Otranto

Pescara

Sibenik

Bari

Dubrovnik

800

Venezia Trieste

RovignoPoDelta N.S.

N.D.

C.D.

C.S.

100

50

Fosse di Pomo

Soglia di Pelagosa


Seawater as thermal source 29

-10

0

10

20

30

40

50

5 10 15 20 25 30

Depth

– [

m]

Seasonal Average Temperature – [°C]

Winter

Summer

Spring

Autumn

AIR

SEAWATER


Data from: UNI 10349:1994 and ISMAR (CNR)

Thanks for your attention! 30

Documents

How heat pumps work: criteria for heat sources evaluation...criteria for heat sources evaluation Paolo Conti and Walter Grassi University of Pisa and UGI Corresponding author email: